Meanwhile mix 1 1/2 sticks of softened unsalted butter in a stand mixer with 1 cup white sugar and 1/3 cup brown sugar (2-3 minutes).

Add 2 eggs, one at a time, as you continue to beat the butter mixture. Then, add vanilla and 1 tsp coffee extract.

In a pyrex measuring cup or bowl, mix together the buttermilk, sour cream, 1/2 cup cold espresso, and 1/4 cup vegetable oil.

With the mixer on low, alternate adding your flour mixture (1/3 at a time) and your buttermilk mixture (1/2 at a time) until you have a smooth batter.

Spoon batter into small cupcake liners in standard cupcake pans, filling each 3/4 of the way full.

Bake for 22-24 minutes until the tops spring back when pressed. Let rest for at least 15-30 minutes.

To make frosting, combine 4 egg whites and 3/4 cup white sugar in a clean stand mixer bowl. Heat about an inch of water in a medium sauce pan over high heat.

Once water is simmering, place your stand mixer bowl on top, being careful not to wet the bottom of the bowl. Cook and stir for 5-10 minutes until the sugar dissolves.

Remove your bowl from heat and transfer to the stand mixer. Whip on high speed for 5-7 minutes, until you have stiff peaks.

On low speed, slowly add 2 sticks of softened butter, 1-2 tablespoons at at time. You should end up with a light and fluffy frosting.

Meanwhile, microwave 1/2 cup cold espresso in a microwave-safe bowl for 1 minute, until nearly boiling. Add your chocolate chips and egg yolks and stir to combine.

On low speed, slowly add your coffee/chocolate mixture to the frosting until combined. Then add 2 tsp coffee extract.

Remove cupcakes from tray. Transfer frosting to a large piping bag with a star nozzle and pipe generous swirls onto each cupcake.