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 cake flour, espresso powder, baking powder, baking soda, 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 sugar and beat on high speed for 2 minutes until creamed together. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) 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 medium-high speed until combined, then beat in the sour cream. Scrape down the sides and up the bottom of the bowl as needed. With the mixer on low speed, add the dry ingredients until just incorporated. With the mixer still running on low, slowly pour in the coffee then the Baileys. Beat just until combined. Do not over-mix. You may need to whisk it all by hand to make sure there are no lumps in the bottom of the bowl.

Pour/spoon the batter into the liners—fill only 2/3 full to avoid spilling over the sides. Bake for 19-22 minutes, or until a toothpick inserted in the center comes out clean. For around 30-36 mini cupcakes, bake for about 11-13 minutes, same oven temperature. Allow the cupcakes to cool completely before frosting.

With a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter on medium speed until creamy, about 2 minutes. Add confectioners’ sugar, espresso powder (if using), Baileys, and coffee. 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 Baileys if frosting is too thick. Add a pinch of salt if frosting is too sweet. (I add and recommend 1/8 teaspoon salt.)

Frost cooled cupcakes and top with sprinkles, if desired. I used Ateco 808 piping tip. Cover and store leftover cupcakes in the refrigerator for up to 5 days.