Preheat the oven to 350° and line a muffin tin with 4 paper liners.

In a small bowl, sift together the flour, cocoa powder, baking soda, baking powder and espresso powder.

In a separate medium bowl, whisk together vigorously the oil, vanilla, and brown sugar.

Using a spatula, add the dry ingredients to the wet in two batches, alternating with the buttermilk.

Divide the batter between the muffin cups and bake 15-17 minutes, or until a toothpick inserted comes out clean.

Let cool completely on a wire rack before proceeding.

Stir together the Nutella and sour cream in a small bowl.

Snip off the corner of a plastic bag or pastry bag and place it in a tall glass. Put a metal round tip in the corner. Add the Nutella-sour cream mixture and squeeze it down to the tip.

Using an upside-down piping tip, gently poke a hole in the top of each cupcake. Twist and pull out the piping tip to remove enough cake in the cupcake to make room for the filling. Divide the Nutella mixture evenly between all 4 cupcakes. Let it firm up for a few minutes in the fridge before frosting.

Next, beat the heavy whipping cream and espresso powder until soft peaks form. Add the cocoa powder and powdered sugar (start with 1 tablespoon, add more to taste).

Frost the cupcakes with the whipped cream mixture using a knife or another pastry bag fitted with a tip for a decorative look.