Prepare Tangzhong: Whisk together 3 tablespoons of bread flour with ½ cup of water in a saucepan over medium heat until it thickens into a paste, then remove from heat and cool.

Mix Dough: Combine 3 cups of bread flour, ½ cup cocoa powder, ¼ cup sugar, 2 teaspoons yeast, and 1 teaspoon salt in a stand mixer. Gradually add 1 cup warm milk, 2 eggs, and tangzhong until a shaggy dough forms.

Incorporate Butter: Slowly add ¼ cup softened butter to the dough while mixing on low. Increase speed and knead for 8-10 minutes until glossy and elastic.

First Rise: Transfer kneaded dough to an oiled bowl, cover, and let rise in a warm area for 60-90 minutes until doubled in size.

Shape Buns: Turn the risen dough onto a floured surface, divide into 12 equal pieces, and shape into ovals. Place on a parchment-lined baking sheet and cover to rest.

Second Rise: Allow shaped buns to rise for 45-60 minutes while preheating the oven to 350°F.

Bake: Brush tops with egg wash and bake for 18-22 minutes until set. Cool on a wire rack.

Prepare Filling: Whip 1 cup cold heavy cream with ¼ cup powdered sugar and 1 teaspoon vanilla until stiff peaks form.

Assemble Buns: Slice cooled buns diagonally, fill with whipped cream, and dust with powdered sugar before serving.