Preheat the oven to 350°F and line a standard cupcake pan with 12 cupcake liners.
In a small bowl whisk together the flour, cocoa powder, baking powder, baking soda and salt. Set aside the dry ingredients.
1 cup (125 g) all-purpose flour, spooned and leveled, 1/4 cup + 1 tablespoon (25 g) Dutch process cocoa powder, 1/2 tsp baking powder, 1/2 tsp baking soda, 1/4 tsp salt
In a large bowl, cream together the butter and granulated white sugar with an electric hand mixer on high speed until fluffy, about 2-3 minutes. (If using the bowl of a stand mixer, use the paddle attachment.)
1/4 cup (56 g) unsalted butter, softened, 3/4 cup (150 g) granulated white sugar
Next add in the egg, egg yolk and vanilla and mix on medium-high speed for 1 to 2 minutes until pale and smooth.
1 egg, at room temperature, 1 egg yolk, at room temperature, 1 tsp vanilla
Add in the sour cream and milk and mix on medium-low. (The mixture will look curdled at this point, but it will pull together once the dry ingredients are added.)
1/4 cup (61 g) sour cream, at room temperature, 1/2 cup (120 ml) whole milk, at room temperature
Add in the flour mixture and mix on low speed, just until the batter is combined and smooth, scraping the sides of the bowl as necessary.
Divide the chocolate cupcake batter among each liner. Fill each cup two-thirds to three-quarters full.
Bake for 17-20 minutes, or until a toothpick/cake tester comes out clean from the centers with just a few moist crumbs.
Let the cupcakes cool in the pan for 10 minutes, then transfer to a cooling rack to cool completely.