Step 1Dissolve the yeast and 1 tablespoon of the sugar in the lukewarm water in the bowl of an electric mixer. Let the mixture sit a few minutes, then add the softened butter and egg yolk and stir with the paddle. Slowly add the flour, salt, remaining sugar and cocoa. Change to the dough hook and knead until the dough becomes uniform.
Step 2Turn the bread onto a floured board, knead the chocolate into the dough and form it into a smooth round. Place the dough in a greased bowl, cover, and let it rise until the dough doubles in bulk, 2 to 2 1/2 hours.
Step 3Punch the dough down, divide in 6 equal parts on a floured board. Let rest for 20 minutes.
Step 4Roll 3 pieces of dough into a sausage about 12 inches long and 2 inches wide. Line up the 3 pieces side by side and braid, then pinch the ends together. Repeat with the other three. Place the two loaves on a baking sheet sprinkled with flour. Cover with a moist towel and let rise for an hour, until the dough doubles again.
Step 5Meanwhile, heat the oven to 375 degrees. Fill a pan with hot water and place it on the lower shelf of the oven. Bake the breads on the upper shelf for 20 minutes. Reduce the temperature to 350 degrees and bake until the bread sounds hollow when tapped, about 20 minutes. Serve warm.