For the butter:
1 lb (2 sticks) unsalted butter, softened
½ cup cocoa powder
⅓ cup bread flour
For the dough:
3 ⅓ cups bread flour, chilled
1 ⅓ cups all-purpose flour, chilled
½ cup cocoa powder
2 tsp kosher salt
8 tbsp (1 stick) unsalted butter, softened
1 ⅓ cup ice-cold water
Bittersweet chocolate bar, chopped
Prep the butter: In a stand mixer with a paddle, combine butter with cocoa powder and bread flour. Transfer to a sheet of plastic wrap and shape into a rectangle that’s roughly 6” x 9”, cover with plastic wrap and chill for 1 hour.
Prep the dough: In a stand mixer with a hook (you can use a spatula in a bowl), add bread flour, all-purpose flour, cocoa powder, salt, and softened butter. Gradually add ice-cold water and mix until a dough forms. Knead dough on a lightly floured surface until smooth. Form dough into a rectangle shape that’s roughly 12” x 18”, cover with plastic wrap and chill for 1 hour.
Lightly flour your surface and lay out the chilled dough. Place butter onto one short end of the rectangle. Fold over the other side over the butter. Pinch dough edges to enclose the butter. Use a rolling pin in a rocking motion to lengthen dough. Roll dough into a long rectangle. Fold dough into thirds, like a letter (use a pastry brush to remove excess flour) and cover in plastic wrap. Chill for 1 hour.
Roll dough into a long rectangle again. Fold dough into thirds, wrap, and chill 30 minutes. Repeat process 3 more times. Chill dough for 1 hour.
Roll dough out again into a rectangle. Slice strips that are the same width as your chocolate batons. Place one piece of chocolate on one end, roll it up once, and then place another chocolate on top and roll up to close. Place on a parchment-lined baking sheet. Repeat with the rest.
Preheat oven to 400F. Brush egg wash on the surface and bake for about 15 minutes or until golden brown. Dust with powdered sugar.