In a stand mixer, combine all dough ingredients: all-purpose flour, bread flour, instant yeast, granulated sugar, salt, vanilla, eggs, milk, neutral oil, and Dutch process cocoa powder. Mix on low speed for about 2 minutes until combined, then knead on medium speed for 8-10 minutes until the dough is smooth and elastic.

Transfer the kneaded dough into a greased bowl, covering it with a clean kitchen towel or plastic wrap. Let the dough rise in a warm, draft-free place until it has doubled in size, about 1 to 2 hours.

Once the dough has risen, punch it down to release air. Divide the dough into equal pieces weighing about 3 ounces each. Shape into smooth balls and place on a parchment-lined tray. Cover and let rise for another 30 to 60 minutes.

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

Gently brush the buns with the egg wash and bake for about 16 minutes, or until golden brown.

In a mixing bowl, whip the heavy cream with cocoa powder, powdered sugar, and a pinch of salt until you reach medium-stiff peaks.

Once the buns are cooled, carefully slice them open and fill with whipped cream.