In a large bowl, beat (using a hand mixer) the cake mix, pudding, eggs, milk, oil, espresso, and liqueur if using on medium speed for 2 minutes. Scrap down the sides midway through mixing.
Transfer the batter to the muffin tins so that they are no more than 2/3 of the way full.
Bake for 20 minutes or until a toothpick comes out clean and the tops puff up slightly.
Allow the cupcakes to cool on a wire rack. Then use a serrated spoon or apple corer to make a small hole in the top of each cupcake.
Fill each cupcake with Nutella.
In a medium bowl beat the butter, vanilla, and liqueur until creamy and smooth.
Add the powdered sugar, half at a time, and beat until fully mixed.
Add Nutella and beat until combined.
While the mixer is running, pour in the heavy cream a tablespoon at a time until the desired consistency is reached.
Transfer to a piping bag with an open star tip to pipe fluffy swirls on top of these delicious Nutella cupcakes.
Top with sprinkles and enjoy!