Preheat the oven to 350ºF.

Sift together the flour, sugar, cocoa powder, baking powder, baking soda, and salt in a large bowl.

In a separate bowl, mix the egg, milk, oil, and vanilla extract.

Pour the wet ingredients into the dry ingredients and whisk until combined.

Add the hot coffee (or hot water) to the batter, which will be very liquidy.

Fill cupcake tins halfway with the batter.

Bake for about 18 minutes until a cupcake springs back when touched, or a toothpick comes out clean.

Remove from the oven and let cool.

Beat the butter in a stand mixer with a paddle attachment or using a hand mixer for 2 minutes.

Add powdered sugar, cocoa powder, Nutella, vanilla, and heavy cream. Start on low speed to combine, then increase to mediumhigh and beat for 2 more minutes.

If the frosting is too runny, add more powdered sugar. If it's too dry, add more heavy cream, one tablespoon at a time.

Use an apple corer, cupcake corer, or spoon to remove the center of each cupcake.

Pipe or spoon Nutella into the center of each cupcake.

Place the frosting in a piping bag with your chosen tip (I used a 2D tip).

Pipe the frosting around the edges of the cupcakes, leaving space in the center. Then, add more Nutella in the center of each cupcake.

Place a hazelnut on top of each cupcake and sprinkle chopped hazelnuts over the frosting.

Store the cupcakes in the fridge in a covered container for up to 4 days.

To freeze, place them in an airtight container and freeze for up to 12 months. To thaw, let them sit in the fridge overnight.