In a stand mixer, combine all-purpose flour, bread flour, instant yeast, granulated sugar, salt, vanilla extract, eggs, milk, neutral oil, and Dutch process cocoa powder. Mix on low until combined, then increase speed to medium and knead for about 8-10 minutes until the dough is smooth and slightly sticky.

Transfer the dough to a lightly greased bowl, covering it with a clean kitchen towel or plastic wrap. Allow it to rise in a warm, draft-free area for 1-2 hours, or until it has doubled in size.

Punch down the dough to release excess air. Divide it into 8-12 equal pieces, rolling each into a smooth ball. Place the shaped buns on a parchment-lined baking sheet and let them rise for another 30-60 minutes.

Preheat your oven to 350°F (175°C) while the buns are rising.

Brush the tops of the risen buns with an egg wash and bake for about 16 minutes, or until they sound hollow when tapped.

In a chilled bowl, whisk together cold heavy whipping cream, cocoa powder, powdered sugar, and a pinch of salt until medium-stiff peaks form.

Slice each cooled bun lengthwise without cutting all the way through. Using a piping bag, fill each bun with the whipped cream.

Lightly dust the filled Chocolate Maritozzi with powdered sugar before serving.