Adjust oven rack to middle position and preheat oven to 350°. Line a cupcake tin with cupcake liners.
Briefly mix together flour, sugar, baking powder, and salt in bowl of standing mixer fitted with paddle attachment (or in a mixing bowl if using a hand mixer).
Add butter, sour cream, milk, egg and egg yolks, and vanilla; beat at medium speed until smooth, about 30 seconds to 1 minute. Scrape down sides of bowl with rubber spatula and mix by hand until smooth, making sure all flour is completely mixed in.
Divide batter evenly among the 12 cups of prepared tin using level ice cream scoop (you will have to scrape the bowl with a rubber spatula to get that 12th cupcake)
Bake until cupcake tops are pale gold and toothpick inserted into center comes out clean, 20 to 24 minutes.
Let cupcakes cool in pan until you can handle them. You may have to gently run a knife around each cupcake if they are sticking and transfer them to wire rack; cool cupcakes completely before frosting.