Preheat the oven to 350°F. Line muffin tins with cupcake liners and set aside.

In a large mixing bowl or the bowl of a standing mixer, combine cake mix, instant pudding, eggs, oil, milk, espresso, and hazelnut liqueur, if using.

Beat on medium-high speed for 2 full minutes, scraping down the sides of the bowl midway through mixing.

Scoop batter into prepared muffin tins, filling each cup no more than ⅔ of the way full.

Bake for 20 minutes or until the cupcakes are puffed and a toothpick inserted in the center of the cupcake comes out with just a few crumbs. Remove from the oven and cool completely.

When the cupcakes are cool, use a serrated spoon to carve out a small hole in the top center of each cupcake. Fill with Nutella.

Beat together the butter, vanilla, and hazelnut liqueur (if using) until smooth and creamy.

Add the powdered sugar, half at a time, and beat until fully mixed. The mixture will be very thick.

Add the Nutella and beat until fully combined.

While the mixer is still running on medium-low speed, pour in the heavy cream, one tablespoon at a time, into the frosting mixture until desired consistency is achieved.

Use a piping bag and open star tip to pipe fluffy swirls of frosting over the filled cupcakes. Sprinkle with chocolate sprinkles.