8 tablespoons unsalted butter (1 stick), room temperature
½ cup sour cream
1 large egg , room temperature
2 large egg yolks , room temperature
1½ teaspoons vanilla extract
Vanilla Buttercream Frosting:
1 cup (8 ounces) unsalted butter, at room temperature
2½ cups powdered sugar
1 tablespoon vanilla extract
Adjust oven rack to middle position; heat oven to 350 degrees F. Line standard muffin/cupcake tin with paper or foil liners.
Whisk together flour, sugar, baking powder, and salt in bowl of standing mixer fitted with paddle attachment. Add butter, sour cream, egg and egg yolks, and vanilla; beat at medium speed until smooth and satiny, about 30 seconds. Scrape down sides of bowl with rubber spatula and mix by hand until smooth and no flour pockets remain.
Divide batter evenly among cups of prepared tin. Bake until cupcake tops are pale gold and toothpick or skewer inserted into center comes out clean, 20 to 24 minutes. Remove the cupcakes from tin and transfer to wire rack; cool cupcakes to room temperature before frosting.
Using the wire whisk attachment of your stand mixer, whip the butter on medium-high speed for 5 minutes, stopping to scrape the bowl once or twice.
Reduce the speed to low and gradually add the powdered sugar. Once all of the powdered sugar is incorporated, increase the speed to medium-high and add the vanilla, mixing until incorporated. Whip at medium-high speed until light and fluffy, about 2 minutes, scraping the bowl as needed.
You can store any unused buttercream in the refrigerator in an airtight container; let it come to room temperature and then give it a quick whip in the mixer before using.