Preheat the oven to 350 degrees. Line a cupcake pan with 12 liners and set aside.
In a small bowl sift together all purpose flour, Dutch process cocoa powder, baking powder, baking soda, salt and espresso powder. Set aside.
1 cup (125 g) all-purpose flour, spooned and leveled, ¼ cup + 1 tablespoon (25 g) Dutch process cocoa powder, ½ teaspoon baking powder, ½ teaspoon baking soda, ¼ teaspoon salt, 1 teaspoon espresso powder
Add granulated sugar and softened butter to a large bowl. Cream together with an electric mixer on high speed for 1-2 minutes until fluffy. (You can also use a stand mixer fit with a paddle attachment.) ¼ cup (56 g) softened, unsalted butter, ¾ cup (150 g) granulated white sugar
Then add in the egg, egg yolk and vanilla and mix on medium speed until light and fluffy, about 1-2 minutes.
1 egg, at room temperature, 1 egg yolk, at room temperature, 1 teaspoon vanilla
Next mix in the milk and sour cream on low speed.
½ cup (120 ml) whole milk, at room temperature, ¼ cup (61 g) sour cream, at room temperature
Add the dry ingredients to the wet ingredients and combine on low speed.
Fill each cupcake liner ¾ of the way full with batter. Bake the cupcakes for 17-20 minutes, or until a toothpick/cake tester comes out clean from the centers.
Let the cupcakes cool in the pan for 10 minutes, then transfer to a cooling rack to cool completely.