01 - In a small bowl, stir the yeast and 1 tbsp of sugar into the lukewarm milk. Let stand for 10 minutes until the mixture becomes foamy and bubbly.
02 - In a large mixing bowl, whisk together the flour, cocoa powder, remaining sugar, and salt until evenly distributed.
03 - Create a well in the center of the dry mixture. Add the egg, vanilla extract, softened butter, and activated yeast mixture. Stir with a wooden spoon until a shaggy dough begins to form.
04 - Knead by hand or with a stand mixer fitted with a dough hook for 8 to 10 minutes until the dough is soft, smooth, and elastic. During the final 2 minutes, incorporate the chopped dark chocolate evenly throughout.
05 - Transfer the dough to a lightly greased clean bowl. Cover with a damp towel or plastic wrap and let rise in a warm spot for 1 to 2 hours until doubled in volume.
06 - Gently punch down the dough and divide into 8 equal portions. Shape each piece into an oval bun and arrange on a parchment-lined baking sheet, spacing them at least 2 inches apart.
07 - Cover the shaped buns loosely with a towel and let proof for 40 minutes until puffed and nearly doubled.
08 - Preheat the oven to 355°F. Whisk together the egg yolk and 2 tbsp of milk. Gently brush the mixture over the tops of each bun for a glossy finish.
09 - Bake on the center rack for 15 to 18 minutes until the buns are slightly firm to the touch and cooked through. Transfer to a wire rack and let cool completely.
10 - In a chilled bowl, whip the heavy cream with powdered sugar and vanilla extract to firm peaks. Gently fold in the melted and cooled dark chocolate until evenly incorporated, being careful not to deflate the cream.
11 - Using a serrated knife, slice each cooled bun lengthwise, leaving one side attached as a hinge. Generously pipe or spoon the chocolate whipped cream into each bun. Dust with powdered sugar if desired and serve immediately.