01 - Stir the yeast and 1 tbsp of sugar into lukewarm milk in a small bowl. 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 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 plastic wrap or a damp towel and let rise in a warm spot for 1 to 2 hours, or until doubled in size.
06 - Punch down the risen dough and divide into 8 equal portions. Shape each piece into an oval bun and arrange on a parchment-lined baking sheet, spacing them 2 to 3 inches apart.
07 - Cover the shaped buns loosely with a damp towel and let rise for 40 minutes until puffy and nearly doubled.
08 - Preheat the oven to 350°F. Whisk together the egg yolk and milk, then brush the mixture evenly over each bun to create a glossy finish.
09 - 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.
10 - Whip the heavy cream with powdered sugar and vanilla extract to stiff 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, 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.