Preheat the oven to 350°F. Line a cupcake pan with 12 liners.
Sift together flour, cocoa powder, baking powder, baking soda, salt, and espresso powder in a small bowl. Set aside.
In a large bowl, cream together butter and granulated sugar until fluffy.
Add egg, egg yolk, and vanilla extract, mixing until light and fluffy.
Mix in milk and sour cream on low speed.
Add dry ingredients to wet ingredients and combine on low speed.
Fill cupcake liners ¾ full with batter. Bake for 17-20 minutes.
Let cupcakes cool in the pan for 10 minutes, then transfer to a cooling rack.