Step 1

Preheat the oven to 400, and line four cups in a muffin pan with paper liners.

Step 2

In a medium bowl, beat with an electric mixer the butter, sugar and vanilla. Beat until light and fluffy, at least 30 seconds.

Step 3

Add the egg white and beat until combined.

Step 4

Next, beat in the sour cream.

Step 5

Finally, sprinkle the flour, baking soda and salt evenly over the batter, and then beat until just combined.

Step 6

Divide the batter between the cupcake liners, and bake on the middle rack for 16-19 minutes. When done, the tops will spring back, and a toothpick inserted will come out clean. (They will brown nicely on top because they're made with butter, not oil).

Step 7

Let the cupcakes cool.

Step 8

Meanwhile, stir together the espresso and Kahlua.

Step 9

Just before serving, poke holes in the cupcakes using a toothpick, and slowly drizzle the espresso mixture into each cupcake.

Step 10

Then, beat together all of the frosting ingredients until light and fluffy.

Step 11

Spoon the frosting into a piping bag fitted with a 1MM Wilton tip and pipe the frosting on the cupcakes (or, just use a butter knife to frost the cupcakes, too!)

Step 12

Top each cupcake with a chocolate-covered espresso bean and serve.