01 - In a small bowl, stir the active dry yeast and 1 tbsp of granulated sugar into the lukewarm milk. Let the mixture stand for 10 minutes until it becomes frothy and activated.
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 the activated yeast mixture. Stir with a wooden spoon until a shaggy dough begins to come together.
04 - Knead by hand on a lightly floured surface or use a stand mixer fitted with a dough hook for 8 to 10 minutes, until the dough becomes soft, smooth, and elastic. During the final minute of kneading, incorporate the chopped dark chocolate until evenly distributed throughout.
05 - Transfer the dough to a clean, lightly oiled bowl. Cover tightly with plastic wrap or a damp kitchen towel. Allow the dough to rise in a warm, draft-free spot for 1 to 2 hours, or until doubled in volume.
06 - Gently punch down the risen dough to release excess air. Divide it into 8 equal portions and shape each into a smooth oval bun. Arrange the shaped buns on a baking tray lined with parchment paper, spacing them about 2 inches apart.
07 - Cover the shaped buns loosely with a damp towel or lightly oiled plastic wrap. Let them proof for 40 minutes until puffed and nearly doubled in size.
08 - While the buns are proofing, preheat the oven to 355°F (180°C). In a small bowl, whisk together the egg yolk and 2 tbsp of milk to create the egg wash.
09 - Brush the tops of the proofed buns evenly with the egg wash. 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 cool completely.
10 - In a chilled mixing bowl, whip the heavy cream with powdered sugar and vanilla extract using an electric mixer until firm, stable peaks form. Gently fold in the melted and cooled dark chocolate using a spatula, 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.