01 - Combine the yeast and 1 tablespoon of sugar with lukewarm milk in a small bowl. Stir gently and let it rest for 10 minutes until the mixture becomes foamy and bubbly.
02 - In a large mixing bowl, whisk together the all-purpose flour, cocoa powder, remaining granulated 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 the activated yeast mixture. Stir with a wooden spoon until a shaggy dough begins to come together.
04 - Turn the dough out onto a lightly floured surface and knead by hand or with a stand mixer fitted with a dough hook for 8 to 10 minutes, until the dough is 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 it rise in a warm spot for 1 to 2 hours, or until doubled in volume.
06 - Gently punch down the risen dough and divide it into 8 equal portions. Shape each piece into an oval bun and arrange them on a parchment-lined baking sheet, leaving space between each for expansion.
07 - Cover the shaped buns loosely with a towel and let them proof for 40 minutes until puffed and nearly doubled.
08 - Preheat the oven to 355°F (180°C). Whisk together the egg yolk and 2 tablespoons of milk to create the egg wash.
09 - Brush the tops of the risen buns evenly with the egg wash. Bake on the center rack for 15 to 18 minutes, until the buns are set and feel slightly firm to the touch. Transfer to a wire rack and cool completely.
10 - Whip the heavy cream with powdered sugar and vanilla extract to stiff peaks using an electric mixer. 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.