01 - In a large mixing bowl, whisk together the flour, cocoa powder, sugar, yeast, and salt until evenly distributed.
02 - Pour in the lukewarm milk, add the softened butter, egg, and orange zest. 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, springy, and no longer sticks to your hands.
04 - Transfer the dough to a lightly greased bowl, cover with a damp towel or plastic wrap, and let it rest in a warm, draft-free spot for 1 to 1½ hours until doubled in volume.
05 - Gently punch down the risen dough and divide it into 8 equal portions. Roll each into an oval bun and arrange on a parchment-lined baking sheet, leaving space between them. Cover loosely and let proof for 30 minutes.
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.
07 - Heat 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 cream with powdered sugar to stiff peaks, then gently fold in the cooled ganache until uniformly blended.
08 - Using a sharp knife, slice each cooled bun lengthwise, leaving one side attached as a hinge. Generously pipe or spoon the chocolate whipped cream into each bun.
09 - Dust the filled maritozzi generously with powdered sugar and serve immediately.