Preheat oven to 350°F. Prepare a cupcake pan with cupcake liners. Place hazelnut flour on a small cookie sheet and place it into the oven to toast for 10-15 minutes, ensuring you stir it every 5 minutes to promote even toasting. Once golden, allow to cool for 10 minutes.

In a large mixing bowl, add the cooled toasted hazelnut flour, AP flour, baking soda, baking powder, and salt. Whisk together to combine. Set aside.

In a small mixing bowl or 2-cup liquid measuring cup, add the hot coffee and cocoa powder. Whisk together until well-combined and smooth. Allow to sit for 2 minutes, then add the cocoa mixture into the mixing bowl with the wet ingredients (eggs, sour cream, etc.), then mix until smooth.

Add the wet ingredients into the bowl with the dry ingredients, and mix until smooth. Pour the cake batter into the prepared cupcake pan, filling 2/3 full, and bake for 20 minutes or until an inserted toothpick comes out clean. Allow to cool completely before frosting.

Add butter to a stand mixer fitted with the whisk attachment. Beat on high until butter is doubled in volume, and looks light and fluffy, about 3-5 minutes. Then add the Nutella and beat until well-combined with the butter.

Turn the stand mixer to low and slowly add the confectioners sugar and cocoa powder. Beat on medium until completely incorporated and smooth.

Add in the heavy cream. Beat until well-combined and fully incorporated.