Preheat the oven to 350 degrees and line a cupcake pan with 12 cupcake liners.
In a small bowl whisk together the flour, cocoa powder, baking powder, baking soda, salt and espresso powder. Then set aside.
1 cup (125 g) all-purpose flour, spooned and leveled, 5 tablespoon (25 g) Dutch process cocoa powder, ½ teaspoon baking powder, ½ teaspoon baking soda, ¼ teaspoon salt, 1 teaspoon espresso powder (optional)
Cream the butter and sugar together in a large bowl with an electric mixer on high speed until fluffy, about 2-3 minutes. (If using a stand mixer, use a paddle attachment.)
¼ cup (56 g) unsalted butter, softened, ¾ cup (150 g) granulated white sugar
Then add in egg, egg yolk and vanilla and mix on medium-high speed until pale in color and smooth, about 1 minute.
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 until combined. The mixture will look curdled but don't worry, it will pull together once the dry ingredients are added in.
¼ cup (60 g) sour cream, at room temperature, ½ cup (120 ml) whole milk, at room temperature
Add the dry ingredients to the wet ingredients and combine on low speed. Scrape the sides and bottom of the bowl as necessary with a rubber spatula.
Divide the batter evenly among the 12 liners, each should be about three-quarters full. Bake the cupcakes for 17-19 minutes.Let the cupcakes cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.