01 - In a large mixing bowl, whisk together the flour, cocoa powder, granulated 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 out onto a lightly floured surface and knead for approximately 10 minutes, working it 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 set in a warm, draft-free spot for 1 to 1 1/2 hours until the dough has doubled in volume.
05 - Gently punch down the risen dough and divide it into 8 equal portions. Roll each piece into an oval bun and arrange on a parchment-lined baking sheet, leaving space between each. Cover loosely and let proof for another 30 minutes.
06 - While the buns proof, preheat the oven to 350°F. 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 1/2 tablespoons of the heavy cream in a small saucepan until steaming. Pour over the chopped dark chocolate and stir until a glossy, smooth ganache forms. Let it 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 the mixture is uniform.
08 - Using a sharp knife, slice each cooled bun lengthwise, leaving one side attached as a hinge. Pipe or spoon a generous amount of chocolate whipped cream into each bun.
09 - Dust the filled maritozzi generously with powdered sugar and serve immediately for the best texture and flavor.