Preheat the oven to 350°F (177°C). Line a 12-cup muffin pan with cupcake liners, and a second pan with 2 liners as this recipe makes about 14 cupcakes. Set aside.
Prepare the dry ingredients: Whisk together the cake flour, espresso powder, baking powder, baking soda, and salt. Set aside.
Using a stand or handheld mixer with a paddle attachment, cream the butter on high speed until smooth and creamy, about 1 minute. Add the sugar and beat on high for 2 minutes until light and fluffy.
Mix in the wet ingredients: Add the egg whites and vanilla extract, beating until combined. Add the sour cream and mix until fully incorporated. Scrape down the sides of the bowl as needed.
With the mixer on low speed, gradually add the dry ingredients, followed by the coffee and Baileys. Mix until just combined—avoid over-mixing.
Fill the cupcake liners about 2/3 full to prevent overflowing. Bake for 19-22 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool completely before frosting.
Prepare the Baileys Coffee Buttercream: Beat the butter on medium speed for 2 minutes until creamy. Add the confectioners’ sugar, espresso powder (if using), Baileys, and coffee. Beat on low speed for 30 seconds, then increase to medium-high speed for 2 minutes until light and fluffy. Adjust consistency as needed by adding more confectioners’ sugar or Baileys. Add a pinch of salt to balance sweetness.
Frost the cooled cupcakes and garnish with chocolate sprinkles if desired.