In a large mixing bowl, whisk together the unsweetened cocoa powder and hot coffee until smooth.

Add the oil, granulated sugar, egg, and vanilla extract to the cocoa mixture. Whisk until fully combined. Then add the sour cream and whisk again until the batter is smooth.

Sift or whisk together the flour, baking powder, baking soda, and salt. Gently fold the dry ingredients into the wet mixture until just combined. The batter will be thin.

Using a large cookie or ice cream scoop, portion the batter evenly into the cupcake liners, filling each about ¾ full.

Bake for 13-18 minutes, or until a toothpick inserted in the center of a cupcake comes out clean. Let the cupcakes cool in the pan for 10 minutes before transferring them to a wire rack to cool completely.

In a large bowl, use a stand mixer with the paddle attachment (or hand mixer) to beat the softened butter on medium speed until creamy. Then beat in the Nutella, vanilla, and salt on medium speed.

With the mixer on low speed, gradually add the powdered sugar, 1 cup at a time, mixing after each addition. Once combined, beat for 1-2 minutes on medium-high speed until the frosting is light and fluffy.

Transfer the Nutella frosting to a piping bag fitted with your favorite piping tip. Pipe swirls onto each cupcake, then garnish with chopped hazelnuts.