Prepare Tangzhong by combining 3 tablespoons of bread flour with ½ cup of water in a saucepan. Heat over medium for about 90 seconds until thickened. Let cool.

Mix Dough by combining 3 cups of bread flour, ½ cup of cocoa powder, ¼ cup of sugar, 2 teaspoons of yeast, and 1 teaspoon of salt in a stand mixer. Add 1 cup of warm milk, 2 eggs, and cooled tangzhong. Mix until shaggy.

Incorporate Butter by gradually adding ¼ cup of softened unsalted butter while kneading on medium speed for 8-10 minutes until smooth and slightly tacky.

First Rise by placing dough in an oiled bowl, covering, and letting it rise in a warm area for 60-90 minutes until doubled.

Shape Buns by deflating the dough, dividing into 10-12 pieces, shaping into balls, then elongating into ovals. Place on lined baking sheet and let rest.

Second Rise by letting buns rise for 45 minutes to 1 hour while preheating the oven to 350°F.

Egg Wash and Bake by whisking one egg with water, brushing the tops, and baking for 18-22 minutes until firm. Cool on a wire rack.

Prepare Filling by whipping 1 cup of cold heavy cream with 2 tablespoons of powdered sugar and 1 teaspoon of vanilla extract until stiff peaks form.

Fill Buns by slicing each bun diagonally and filling with whipped cream, dusting tops with powdered sugar before serving.