Spray a muffin pan with cooking spray (or use cupcake liners) and preheat oven to 350 degrees F.

In a medium-sized mixing bowl, whisk together the following dry ingredients: flour, baking soda, baking powder, cocoa powder, and salt. Make sure they are well combined. Set aside the dry ingredients mixture.

In a large bowl, whisk together your egg and sugar.

Add vegetable oil to the sugar and egg mixture and mix some more.

Alternate between adding milk and the dry ingredient that you set aside. Mix thoroughly each time.

Once all of your ingredients are well combined, mix in the hot water.

Scoop your batter into sprayed muffin pan or cupcake liners, filling each section about 2/3 full.

Bake for 15 to 17 minutes. Allow cupcakes to fully cool.

Once the cupcakes are cool, carve out the top portion of each cupcake, about ½ an inch in size and ½ an inch deep. You can also use a cupcake corer if you prefer.

Fill each cupcake hole with about 1 tbsp of Nutella (or other chocolate hazelnut spread). You will be cover this up later with frosting, though you can try to replace part of the removed portion of the cupcake, if you prefer.

Using an electric mixer, beat the butter until smooth.

Add powdered sugar, milk, and cocoa powder to the butter. Start on slow speed and gradually increase to high.

Once your frosting ingredients are well-combined and form semi-stiff peaks, scoop in a piping bag. (In the pictures here, a Wilton icing tip #1M was used.)

Frost each cupcake.

If you want to add pistachios as a garnish, place your pistachios in a plastic sandwich bag. Crush the pistachios into small pieces with a rolling pin. (This part can be done earlier, of course, so your crushed pistachios are ready to go right after you frost your cupcakes.)

Sprinkle some crushed pistachios on each cupcake.