Question: What is the artist's most mysterious and challenging painting and when was it painted?
About the article "Dora Maar au Chat", section "Descriptions of painting".

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View of Delft is an oil painting by Johannes Vermeer.
Painted ca.
1660-1661, the painting of the Dutch artist's hometown is among his most popular.
Painted at a time when cityscapes were not commonplace, it is one of three known paintings of Delft by Vermeer, along with "The Little Street" and the lost painting "House Standing in Delft".
The use of pointillism in the work suggests it postdates "The Little Street".
In 2011, the painting was featured on gold and silver commemorative coins issued by the Royal Dutch Mint.
The painting features in Marcel Proust's great novel In Search of Lost Time. 
Picasso painted "The Actor" during the winter of 1904–1905 when he was 23 years old.
The painting is a work of the artist's Rose Period when he changed his painting style from the downbeat tones of his Blue Period to warmer and more romantic hues.
Picasso painted "The Actor" on a the reverse side of a landscape painting by another artist because he could not afford new canvases at the time.
The painting currently resides in the Metropolitan Museum of Art in New York City.
It was donated to the museum in 1952 by automobile heiress Thelma Chrysler Foy, daughter of Walter Chrysler, the founder of the Chrysler automobile company.
Experts estimate that the painting, which is one of the largest from Picasso's Rose Period, is worth more than US$100 million. 
Self Portrait (or The Large Self-Portrait) is an oil on canvas painting by the Dutch artist Rembrandt.
Painted in 1652, it is one of at least 40 self-portraits painted by the artist, and was the first he had painted since 1645.
In composition it is different from his previous self-portraits, depicting the painter in a direct frontal pose, hands on his hips, and with an air of self-confidence.
It was painted the year that his financial difficulties began, and breaks with the sumptuous finery he had worn in previous self-portraits.
Art historian Christopher White has called it "one of the most magisterial and sombre of these (late) pictures".
It is in the Kunsthistorisches Museum in Vienna.
The freely painted clothing includes a brown robe that was most likely casual working attire, secured with a sash, over a black doublet with an upturned collar.
A drawing from c. 1650 shows Rembrandt in much the same pose and attire, and features an inscription, though not by the artist's hand, stating that these were the artist's studio clothes.
In the drawing Rembrandt is seen wearing a top hat, while in the painting he wears a black beret derivative of artists' portraits of the 16th century.
Following a period of seven years when he painted no self-portraits, focusing instead on landscapes and intimate domestic subjects, the Vienna "Self Portrait" inaugurated a prolific stretch in which Rembrandt painted an average of one self-portrait a year until his death in 1669.
Contrary to the popular understanding that these paintings primarily represented a deeper personal interest in self-depiction, Ernst van de Wetering has proposed that they were painted specifically for connoisseurs who collected self-portraits by prominent artists.
As in other late portraits and self-portraits by Rembrandt, a painted underlayer shows through in areas of shadow, here particularly in the eye sockets and beneath the moustache.
Microscopic analysis has revealed that this is not the painted ground layer, which is a similar gray color, but a separate underlayer of paint.
This local imprimatura, used in preparation for specific areas of the painting, was also practiced by Vermeer, and its purpose is not fully understood.
A strong similarity has been noted to the self-portrait of c. 1655, also in the Vienna Kunsthistorisches Museum.
The later work shares the frontal angle, lighting, and informal attire of the larger painting, though the artist's face appears older.
A painting of Rembrandt' son Titus van Rijn, by another artist in the style of Rembrandt, duplicates the pose and positioning of the figure in the "Self Portrait". 
"Dora Maar au Chat" presents the artist's most mysterious and challenging mistress regally posed three-quarter length in a large wooden chair with a small black cat perched behind her in both an amusing and menacing attitude.
The faceted planes of her body and richly layered surface of brushstrokes impart a monumental and sculptural quality to this portrait.
The painting is also remarkable for its brilliance of colour and the complex and dense patterning of the model's dress.
The powerful figure is set in a dramatic, yet simple setting composed of a vertiginously inclined plane of wooden floorboards and shallow interior space that is arranged in a manner reminiscent of Picasso’s earliest manipulations of space in a cubist manner.
"Dora Maar au Chat" is one of Picasso’s most valued depictions of his lover and artistic companion.
Their partnership had been one of intellectual exchange and intense passion—Dora was an artist, spoke Picasso’s native Spanish, and shared his political concerns.
She even assisted with the execution of the monumental "Guernica" and produced the only photo-documentary of the work in progress.
She was an intellectual force – a characteristic that both stimulated and challenged Picasso and her influence on him resulted in some of his most powerful and daring portraits of his 75-year career.
Among the best of them are the oils completed during the late 1930s and early 1940s, when Picasso’s art resonated with the drama and emotional upheaval of the era and which Dora came to personify.
The luminous "Dora Maar au Chat" was painted in 1941, at the beginning of the Second World War in France .
Maar was one of the most influential figures in Picasso’s life during their relationship and she also became his primary model.
By the time he painted the present picture he had incorporated Dora Maar’s image into countless versions of this motif.
During the occupation of Paris by the Nazis, and as tension mounted in their relationship, the artist would express his frustration by furiously abstracting her image, often portraying her in tears.
While the present portrait might seem a departure from Picasso's more hostile depictions of this model, it may be one of his most brilliant and biting provocations of his "Weeping Woman".
Picasso once likened Maar’s allure and temperament to that of an “Afghan cat”, and the cat in this picture is laden with significance.
In the history of art, the pairing of cats and women was an allusion to feminine wiles and sexual aggression, as exemplified in Manet’s notorious "Olympia".
It is also interesting to consider that the artist has paid particular attention to the sharp, talon-like nails on the long fingers of his model.
In life Maar’s well-manicured hands were one of her most beautiful and distinctive features, and here they have taken on another, more violent characteristic.
In addition to being a rare, three-quarter length portrait of Dora Maar, the present work is also a generous and painterly composition with an extraordinary attention to detail.
The artist used an extraordinarily vibrant palette in his rendering of the angles of the chair and the patterning of Maar’s dress.
The most embellished and symbolic element of the sitter’s wardrobe in this picture is her hat, Maar’s most famous accessory and signifier of her involvement in the Surrealist movement.
Ceremoniously placed atop her head like a crown, it is festooned with colourful plumes and outlined with a band of vibrant red.
Larger than life, an impression enhanced by her vibrant body that cannot be confined by the boundaries of the chair, Maar looms in this picture like a pagan goddess seated on her throne.
When the MNAC opened in 2004, there were some gaps in content especially with respect to modern artists.
One of the most absent was Picasso.
The work entered the museum in 2007, one of a set of 8 pieces that are part of a group and fills many gaps in the artist's career.
Details of the work: Oil on canvas Painted in Paris, on December 4, 1937.
The painting is not signed but dated as "4D37" in the upper right corner.
It is a deposit of State since 2007, the company Albertis Foundation joined the museum MNAC reference number 214090.