In a medium-sized mixing bowl or bowl of a stand mixer, mix together sugar, cake flour, baking powder, baking soda, and salt.
Add butter and mix on medium-low speed for three minutes. Because there is so little butter, you'll end up with a very fine crumb texture.
In a small mixing bowl, whisk together eggs, sour cream, oil, and vanilla extract until smooth.
Add the egg mixture to the flour mixture and beat on medium speed until just combined.
In a small bowl or liquid measuring cup, whisk together warm milk, Kahlua, Baileys Irish Cream, and instant coffee until the coffee is dissolved.
Slowly add liquid to mixing bowl and mix on low speed until just combined. The batter will be liquidy. (Don't worry, you didn't do anything wrong. It's supposed to be that way.)
Fill cupcake liners just over 1/2 full.
Bake for 14 minutes and then test to see if they are done. They are done when a toothpick comes out without wet batter stuck to it. If they are not done, test again in two minutes. If they are still not done, test again in another two minutes.
When the cupcakes are done, remove them immediately from the tins and leave them on a cooling rack (or just on your counter if you don’t own a cooling rack) to cool.
Baileys Frosting Instructions
Beat butter on high speed with an electric mixer for about three minutes until light and airy.
Mix in powdered sugar a little bit at a time.
Add Kahlua, Irish Cream, and salt and continue to beat for another minute.
Pipe frosting onto cooled cupcakes. Optionally drizzle with chocolate syrup and top with chocolate shavings.
Notes
Use cake flour for a lighter crumb.
Be careful not to over-mix the batter.
Use room temperature ingredients – everything will mix together more smoothly that way.
Because everyone’s oven is just a bit different and your liners may be a slightly different size than mine, I highly recommend that you bake a test cupcake before baking your whole batch of cupcakes. Fill one liner halfway and see how much it rises during the bake.
I keep cooled, frosted cupcakes in large Tupperware-style containers. Food safety experts recommend that cupcakes with whipped cream, buttercream, cream cheese, or ganache frostings should be refrigerated. While I always keep cupcakes frosted with whipped cream or ganache in the refrigerator, I often keep my buttercream and cream cheese cupcakes on the counter. Do so at your own risk.
If you know you’ll need to store your cupcakes for more than 3-4 days, these cupcakes can be frozen without a problem. Be sure to store the cupcakes in an airtight container to avoid freezer burn.
Always return refrigerated or frozen cupcakes to room temperature before serving.
Double the amount of frosting listed if you prefer a cupcake with an extra large swirl as shown in the photos.