01 - In a small bowl, stir the yeast and 1 tbsp of sugar into the lukewarm milk. Let the mixture stand for 10 minutes until it becomes foamy and active.
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 comes together.
04 - Knead by hand or with a stand mixer fitted with a dough hook for 8 to 10 minutes, until the dough becomes soft, smooth, and elastic. During the final 2 minutes, fold in the chopped dark chocolate until evenly distributed throughout.
05 - Transfer the dough to a lightly oiled clean bowl, cover with plastic wrap or a damp towel, and let it rise in a warm spot for 1 to 2 hours until doubled in volume.
06 - Punch down the dough gently and divide it into 8 equal portions. Shape each piece into an oval bun and arrange them on a baking tray lined with parchment paper, spacing them about 2 inches apart.
07 - Cover the shaped buns loosely with a towel and let them proof for 40 minutes until puffy and nearly doubled.
08 - Preheat the oven to 355°F. Whisk together the egg yolk and milk, then brush the mixture evenly over the tops of each bun for a glossy finish.
09 - Bake for 15 to 18 minutes until the buns are slightly firm to the touch and cooked through. Transfer to a wire rack and let them cool completely.
10 - Using an electric mixer, whip the heavy cream with powdered sugar and vanilla extract to firm peaks. Gently fold in the melted and cooled dark chocolate until fully incorporated, being careful not to deflate the cream.
11 - 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 additional powdered sugar if desired and serve immediately.