Preheat the oven to 350 degrees Fahrenheit and line a cupcake pan with cupcake liners.
In a small bowl, whisk together flour, black cocoa powder, baking powder, baking soda, salt, and espresso powder. Set aside.
In a large bowl, cream together butter and sugar with an electric mixer until fluffy, about 2-3 minutes.
Add egg, egg yolk, and vanilla, mixing until pale and smooth, about 1 minute.
Mix in milk and sour cream on low speed until combined.
Add dry ingredients to wet ingredients and combine on low speed.
Divide batter evenly among cupcake liners, filling each about three-quarters full. Bake for 17-19 minutes.
Let cupcakes cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.