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 along with the softened butter, egg, and orange zest. Stir vigorously until a sticky, cohesive dough comes together.
03 - Turn the dough onto a lightly floured surface and knead for approximately 10 minutes until it becomes smooth, elastic, and slightly tacky to the touch.
04 - Transfer the dough to a greased bowl, cover with a damp towel or plastic wrap, and let it rest in a warm, draft-free spot for 1 to 1 1/2 hours until doubled in volume.
05 - Gently deflate the dough and divide it into 8 equal portions. Shape 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. Bake the buns for 18 to 20 minutes until just set and slightly springy to the touch. Transfer to a wire rack and cool completely before filling.
07 - Heat about 3 1/2 tablespoons of the heavy cream in a small saucepan until steaming. Pour it over the finely chopped dark chocolate and stir until a smooth ganache forms. Allow to cool to room temperature. Whip the remaining heavy cream with the powdered sugar to stiff peaks, then gently fold in the cooled ganache until uniformly combined.
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 opening.
09 - Dust the filled maritozzi generously with powdered sugar and serve immediately.