Complex sentence: They are a variation of croissants with the extremes put together, found both in Uruguay and Argentina, leaving some space in the middle for a filling (pastry cream, dulce de membrillo or dulce de leche).
Split into simpler sentences: They are a variation of croissants found both in Uruguay and Argentina. The the ends are joined together, leaving some space in the middle for a sweet filling (pastry cream, dulce de membrillo or dulce de leche).