Preheat the oven to 350°F (177°C). Line a 12-count muffin pan with cupcake liners. Line a second pan with 2 liners—this recipe makes about 14 cupcakes. Set aside.

Whisk the flour, baking powder, and salt together. Set aside.

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter on high speed until smooth and creamy—about 1 minute. Add the granulated sugar and beat on high speed for 2 minutes until creamed together. Scrape down the sides and up the bottom of the bowl with a silicone spatula as needed. Add the egg whites and vanilla extract. Beat on high speed until combined, then beat in the sour cream. Scrape down the sides and up the bottom of the bowl as needed so everything is incorporated.

Stir the espresso powder and hot water together until smooth, then stir this into the milk. Set aside for a second.

With the mixer running on low speed, add the dry ingredients until just incorporated. With the mixer still running on low, slowly pour in the espresso/milk mixture until combined. Do not overmix. You may need to whisk it all by hand to make sure there are no lumps at the bottom of the bowl. The batter will be on the thin side.

Pour/spoon the batter into the liners—fill only halfway with batter to avoid spilling over the sides. Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean. Allow to cool completely before frosting.

In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until smooth, about 2 minutes. Stop the mixer and add confectioners’ sugar, Nutella, heavy cream, and vanilla extract. Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 2 full minutes. Add up to 1/2 cup more confectioners’ sugar if frosting is too thin or another Tablespoon of cream if frosting is too thick. Add a pinch of salt if frosting is too sweet. (I always add 1/8 teaspoon salt.)

Frost and decorate cooled cupcakes. I used a Wilton 1M piping tip for these photos. (Another favorite is this Wilton #12 round tip.)

Serve immediately. Store leftover cupcakes in the refrigerator for up to 5 days. I recommend a cupcake carrier for storing and transporting decorated cupcakes.