Preheat Oven and Prep Pan:
Set oven to 350°F (175°C). Place liners in a 12-cup muffin pan.Sift Dry Ingredients:
In a small bowl, sift flour, cocoa powder, baking powder, baking soda, salt, and espresso powder if using. Reserve.Cream Butter and Sugar:
In a large bowl, beat sugar and softened butter with an electric mixer on high until fluffy, 1-2 minutes. A stand mixer with paddle attachment also works.Beat in Eggs and Vanilla:
Add egg, egg yolk, and vanilla; beat on medium until light and airy, about 1-2 minutes.Mix in Milk and Sour Cream:
On low speed, mix in milk and sour cream.Combine Wet and Dry Mixtures:
Add dry mixture to wet mixture; blend on low just until incorporated.Fill Liners and Bake:
Fill each liner to 3/4 capacity. Bake 17-20 minutes, until a toothpick inserted into the center emerges clean.Cool Cupcakes in Pan:
Cool in pan for 10 minutes, then move to a wire rack to cool fully.