01 - Stir the yeast and 1 tbsp sugar into lukewarm milk in a small bowl. Let stand for 10 minutes until the mixture becomes foamy and bubbly.
02 - Whisk together the flour, cocoa powder, remaining sugar, and salt in a large mixing bowl 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 comes together.
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. Fold in the chopped dark chocolate during the final 2 minutes of kneading.
05 - Transfer the dough to a clean, lightly oiled 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 - Punch down the risen dough and divide into 8 equal portions. Shape each piece into a plump oval bun and arrange on a parchment-lined baking sheet, spacing them about 2 inches apart.
07 - Cover the shaped buns loosely and let them proof for 40 minutes until puffed and nearly doubled.
08 - Preheat the oven to 355°F. Whisk the egg yolk with 2 tbsp milk and brush the mixture evenly over each bun for a glossy finish.
09 - Bake on the center rack for 15 to 18 minutes until the tops are set and a toothpick inserted into the center of a bun comes out clean. Transfer to a wire rack and cool completely.
10 - Whip the heavy cream with powdered sugar and vanilla extract to firm, stable peaks. Gently fold in the melted and cooled dark chocolate until evenly incorporated, being careful not to deflate the cream.
11 - Slice each cooled bun lengthwise with a serrated knife, leaving one side attached as a hinge. Pipe or spoon the chocolate cream generously into each bun. Dust with powdered sugar if desired and serve immediately.