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If using a mixer, insert the dough hook and use the low speed for about 2 minutes. Stop, scrape the sides into the dough, and mix for 3 more minutes on medium. Cut the butter into 1 centimeter (0.4in)-thick (just under 1/2in) pieces and place between two sheets of parchment paper. Roll and pound until you have a 20x20cm (8inx8in) square. You may need to cut off and move pieces to make the square even. Place the butter so that one corner of dough sticks out from each of the four sides of the butter. Be sure to firmly seal all edges. Repeat twice more so that butter and dough are thoroughly layered. They should be larger (though not doubled in size) when you're done. If you have a convection oven, it should be 205 C (400 F). If they darken too quickly, reduce temperature 5 degrees C or 10 degrees F.
Mix all dough ingredients in a large bowl. Move the dough onto a floured dinner plate and flour the top. Cover with cling wrap and refrigerate overnight. Make the butter layer. Place butter layer in refrigerator. Place refrigerated dough on a floured surface and roll into a 25x25cm (10x10in) square. Center butter layer over dough at an angle. Fold the corners of dough up around the butter and roll shut with a rolling pin. Gently roll the dough into a rectangle, then fold in thirds like a letter. Wrap dough with cling wrap, place on a flat sheet, and refrigerate overnight. Place refrigerated dough on a floured surface and roll into a 25x37cm (10x15in) square. Cut the dough crosswise into 5-cm (~2in) strips. Chop chocolate into 3x3-cm (~1x1in) chunks. Place one piece of chocolate on either end of a dough strip. Roll both ends of dough inwards so that they meet in the center. Repeat on remaining dough strips. Mix egg with 4 milliliters (0.14floz) (1tsp) water and brush over all dough rolls. Set rolls in a warm area and wait 1 1/2 to 2 hours. Re-brush with egg wash. Place rolls a conventional oven at 218 C (425 F). Bake for 10 minutes, then swap/rotate pans. Bake 8 to 10 more minutes until browned.