01 - In a small bowl, combine the active dry yeast with 1 tbsp of sugar and lukewarm milk. Stir gently and let the mixture rest for 10 minutes until it becomes frothy and bubbly.
02 - In a large mixing bowl, whisk together the all-purpose 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 come together.
04 - Turn the dough 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, soft, 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 plastic wrap or a damp towel 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 damp towel and let them rise for 40 minutes until puffed and nearly doubled.
08 - Preheat the oven to 355°F. Whisk the egg yolk with 2 tbsp of milk and brush the mixture evenly over each bun. Bake for 15 to 18 minutes until the tops are set and the buns sound slightly hollow when tapped. Transfer to a wire rack and cool completely.
09 - In a chilled bowl, whip the heavy cream with powdered sugar and vanilla extract until firm peaks form. Gently fold in the melted and cooled dark chocolate until evenly incorporated, being careful not to deflate the cream.
10 - 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 additional powdered sugar if desired and serve immediately.