Prepare Streusel Crumb: In a mixing bowl, combine plain flour, sugar, cocoa powder, and room temperature butter. Use your fingers to crumble the mixture until it resembles coarse crumbs, then chill it in the refrigerator.

Activate Yeast: In another bowl, whisk together lukewarm milk, active dry yeast, and castor sugar. Let it sit for about 15 minutes, until it becomes foamy and bubbly—a sign that the yeast is alive and ready.

Mix Dry Ingredients: In a separate bowl, combine strong bread flour, salt, and instant coffee. Stir together to evenly distribute the ingredients, enhancing the dough's flavor.

Knead Dough: Pour the activated yeast mixture into the dry ingredients, add the egg, and mix until combined. Knead the dough on a lightly floured surface for about 10 minutes until it’s smooth and elastic, passing the windowpane test.

First Rise: Place the kneaded dough in a greased bowl, cover it with a damp cloth, and let it rest in a warm place for 1 hour, or until it has doubled in size.

Shape Buns: After the dough has risen, gently punch it down. Divide the dough into equal portions, shape them into balls, then flatten and roll them out. Let them rise again for 30 minutes.

Preheat Oven: While the buns are rising, preheat your oven to 180°C (350°F) to ensure it's hot and ready for baking.

Prepare for Baking: Brush each bun with the egg wash mixture prepared from 1 egg and 1 teaspoon of water. Sprinkle the chilled chocolate crumble generously on top.

Bake: Place the buns in the preheated oven and bake for 15 minutes or until they're golden brown. Allow them to cool slightly on a wire rack.

Make Mascarpone Filling: In a mixing bowl, whip the heavy cream with icing sugar and vanilla extract until stiff peaks form. Gently fold in the mascarpone cheese to create a smooth, creamy filling.

Assemble Buns: Once the buns have cooled, cut a slit in each bun and generously fill it with the mascarpone cream. Dust with cocoa powder for an elegant finish.