01 - In a large mixing bowl, whisk together the flour, cocoa powder, sugar, yeast, and salt until evenly distributed.
02 - Add the lukewarm milk, softened butter, egg, and orange zest to the dry mixture. Stir with a wooden spoon or dough hook until a sticky, cohesive dough forms.
03 - Turn the dough onto a lightly floured surface and knead for approximately 10 minutes, until the dough becomes smooth, supple, and springs back when gently pressed.
04 - Transfer the dough to a lightly greased bowl, cover with a clean kitchen towel or plastic wrap, and let it rest in a warm, draft-free spot for 1 to 1½ hours, or until doubled in volume.
05 - Gently punch down the risen dough and divide it into 8 equal portions. Shape each piece into an oval bun and arrange on a parchment-lined baking sheet, leaving space between each. Cover loosely and let rise for another 30 minutes until puffy.
06 - Preheat the oven to 350°F (180°C). Bake the buns for 18 to 20 minutes, until just set and slightly firm to the touch. Transfer to a wire rack and cool completely before filling.
07 - Heat about 3 tablespoons of the heavy cream in a small saucepan until steaming but not boiling. Pour over the chopped dark chocolate and stir until a smooth ganache forms. Let cool to room temperature. Whip the remaining heavy cream with the powdered sugar to stiff peaks, then gently fold in the cooled chocolate ganache until uniformly combined.
08 - Using a sharp serrated knife, slice each cooled bun lengthwise, leaving one side intact as a hinge. Generously pipe or spoon the chocolate whipped cream into the opening.
09 - Dust the filled maritozzi generously with powdered sugar and serve immediately for the best texture and flavor.