01 - In a small bowl, stir the active dry yeast and 1 tbsp of granulated sugar into the lukewarm whole milk. Let the mixture stand for 10 minutes until it becomes foamy and bubbly, confirming the yeast is alive and active.
02 - In a large mixing bowl, whisk together the all-purpose flour, unsweetened 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 unsalted 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 becomes soft, smooth, and elastic. During the final 2 minutes of kneading, incorporate the finely chopped dark chocolate evenly throughout.
05 - Transfer the dough to a clean, lightly oiled bowl. Cover with a damp towel or plastic wrap and let rise in a warm, draft-free spot for 1 to 2 hours, or until the dough has 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 baking tray lined with parchment paper, leaving space between each for expansion.
07 - Cover the shaped buns loosely with a towel and let them rise for 40 minutes until puffy and nearly doubled in size.
08 - Preheat the oven to 355°F. In a small bowl, whisk together the egg yolk and 2 tbsp of milk. Brush the mixture gently over the tops of each risen bun for a glossy, golden finish.
09 - Bake on the center rack for 15 to 18 minutes, until the buns are slightly firm to the touch and cooked through. Transfer to a wire rack and let cool completely before filling.
10 - In a chilled mixing bowl, whip the heavy cream with powdered sugar and vanilla extract using an electric mixer until firm peaks hold their shape. 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 additional powdered sugar if desired and serve immediately.