01 - Combine the active dry yeast and 1 tbsp of granulated sugar with lukewarm milk in a small bowl. Allow the mixture to rest for 10 minutes until it becomes foamy and bubbly, confirming the yeast is 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 butter, and activated yeast mixture. Stir with a wooden spoon until a shaggy dough begins to form.
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. Work until the dough becomes smooth, soft, and elastic. Incorporate 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, draft-free 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 a smooth oval bun and arrange them on a parchment-lined baking tray, leaving space between each for expansion.
07 - Cover the shaped buns loosely with a damp towel or oiled plastic wrap. Allow them to proof for 40 minutes until puffed and nearly doubled in size.
08 - Preheat the oven to 355°F. Whisk together the egg yolk and milk to create an egg wash. Brush the tops of each bun evenly with the mixture for a glossy, golden finish.
09 - Bake on the center rack for 15 to 18 minutes, until the buns feel slightly firm to the touch and are cooked through. Transfer to a wire rack and let them cool completely before filling.
10 - Using an electric mixer, whip the heavy cream with powdered sugar and vanilla extract to firm, stiff peaks. Gently fold in the melted and cooled dark chocolate using a spatula until uniformly combined, taking care 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.