Mocha Cupcakes:Preheat the oven to 350°F (176°C) and prepare a cupcake pan with cupcake liners.
In a large mixing bowl, cream the butter and sugar until light in color and fluffy, about 3 to minutes. Add the sour cream and vanilla extract and mix until well combined. Add 1 of the egg whites and mix until well combined. Add the remaining 2 egg whites and mix until well combined. Scrape down the sides of the bowl as needed to be sure all the ingredients are well incorporated.
Combine the flour, baking powder and salt in a medium bowl.
Combine the hot water and espresso coffee powder in a small measuring cup, then add the milk. Add half of the flour mixture to the batter and mix until well combined. Add the milk mixture and mix until well combined. Add the remaining flour mixture and mix until well combined. Scrape down the sides of the bowl as needed to be sure all the ingredients are well incorporated.
Fill the cupcake liners about halfway with batter. Bake the cupcakes for 16 to 18 minutes (mine were done at 14 minutes, so check early on!), or until a toothpick inserted comes out with a few crumbs. Remove the cupcakes from the oven and allow them to cool for 2 to 3 minutes, then transfer them to a cooling rack to finish cooling.
Frosting:To make the frosting, beat the butter and shortening in a large bowl until smooth. Add 2 cups (230 g) of the powdered sugar and mix until smooth. Add the Nutella and water or milk and mix until well combined. Add the remaining 2 cups (230 g) powdered sugar and mix until smooth.
Cupcakes can be stored in the refrigerator or frozen, or kept at room temperature for up to 2-3 days..
Notes
*I used 4 tablespoons 0% Greek yogurt in place of the sour cream and 2 whole eggs instead of egg whites (because I hate having halves of things!). The cupcakes turned out great! *I mixed the first batch of wet ingredients, then added the dry ingredients right to the same bowl and mixed. I ended with the coffee and milk mixture (this is just the way my mom taught me to bake -- we're not into dirtying extra bowls!) *I drizzled mine with Nutella that had been microwaved on high for 20 seconds first. It makes them look pretty and is super simple! (Plus, Nutella)