Prepare the Cupcake Batter: Preheat your oven to 350°F (or the temperature listed on your cake mix box). Line a standard 12-cup muffin tin with cupcake liners. In a large mixing bowl, prepare the chocolate cupcake batter according to the directions on the box. This usually involves mixing together the dry cake mix, eggs, oil, and water. Use a hand mixer or whisk to blend until smooth.

Swirl in the Nutella: Spoon the prepared batter into each cupcake liner, filling them about ⅔ full. Warm the Nutella slightly if needed (10 seconds in the microwave works great) to make it easier to swirl. Add about ½ tablespoon of Nutella to the top of each batter-filled cup. Use a toothpick or skewer to gently swirl the Nutella into the batter, creating a marbled effect without fully mixing it in.

Bake the Cupcakes: Place the muffin tin in the preheated oven. Bake according to the box instructions typically 18 to 20 minutes. To check for doneness, insert a toothpick in the center. It should come out with just a few moist crumbs. Remove cupcakes from the oven and let them cool in the pan for about 5 minutes, then transfer to a wire rack to cool completely.

Make the Nutella Buttercream: While the cupcakes are cooling, make the frosting. In a bowl, combine 1 cup of buttercream frosting with ½ cup of Nutella. Stir or beat until smooth and creamy. Taste and adjust add a little more Nutella if you want it extra rich!

Frost and Garnish the Cupcakes: Once cupcakes are completely cooled, transfer the frosting into a piping bag fitted with a large star tip (or use a zip-top bag with the corner cut off). Pipe generous swirls onto each cupcake.

Sprinkle crushed hazelnuts on top if desired.