01 - In a large mixing bowl, whisk together the flour, cocoa powder, granulated sugar, active dry 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 comes together.
03 - Turn the dough onto a lightly floured surface and knead firmly for about 10 minutes, until the dough becomes smooth, supple, and springs back when pressed.
04 - Transfer the dough to a greased bowl, cover tightly with plastic wrap or a damp towel, and set in a warm, draft-free spot for 1 to 1½ hours until doubled in volume.
05 - Punch down the risen dough and divide into 8 equal portions. Roll each into a plump oval shape and arrange on a parchment-lined baking sheet, spacing them apart. Cover loosely and let proof for 30 minutes until puffed.
06 - Preheat the oven to 350°F (180°C). Bake the buns for 18 to 20 minutes until just set and the tops feel firm to the touch. Transfer to a wire rack and cool completely.
07 - Gently heat about 3 tablespoons of the heavy cream in a small saucepan until steaming. Pour over the chopped dark chocolate and stir until a glossy ganache forms. Let cool to room temperature. Whip the remaining heavy cream with powdered sugar to stiff peaks, then fold in the cooled ganache until uniformly blended.
08 - Using a sharp serrated 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.