01 - Combine active dry yeast and 1 tsp of sugar in lukewarm whole milk. Stir gently and allow the mixture to rest for 10 minutes until the surface becomes frothy and bubbly, indicating active fermentation.
02 - Sift together all-purpose flour, unsweetened cocoa powder, remaining granulated sugar, and salt in a large mixing bowl until evenly distributed and no lumps remain.
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 smooth and elastic. During the final 2 minutes, fold in the chopped dark chocolate until evenly distributed throughout the dough.
05 - Transfer the dough to a clean, lightly oiled bowl. Cover tightly with plastic wrap or a damp kitchen towel and let rise in a warm, draft-free spot for 1 to 2 hours, or until doubled in volume.
06 - Gently punch down the risen dough and divide into 8 equal portions. Shape each piece into an elongated oval, tucking the seams underneath. Arrange on a parchment-lined baking sheet with ample spacing between each bun.
07 - Cover the shaped buns loosely with a damp towel or oiled plastic wrap. Allow to proof for 40 minutes until puffy and nearly doubled in size.
08 - Preheat the oven to 355°F (180°C). Whisk together the egg yolk and milk to create the egg wash. Gently brush the tops of each risen bun with the mixture for a glossy, golden-brown finish.
09 - Bake on the center rack for 15 to 18 minutes, until the buns feel slightly firm to the touch and a toothpick inserted comes out clean. Transfer to a wire rack and cool completely before filling.
10 - Using an electric mixer, whip the heavy cream with powdered sugar and vanilla extract to stiff peaks. Gently fold in the melted and cooled dark chocolate using a spatula, maintaining a light and airy consistency. Refrigerate until ready to use.
11 - Using a serrated knife, slice each cooled bun lengthwise, leaving one side attached as a hinge. Pipe or spoon the chocolate whipped cream generously into each bun. Dust with additional powdered sugar if desired and serve immediately.