Preheat oven to 350°F. Line a standard 12-cup muffin pan with cupcake liners.

In a medium bowl, whisk together flour, baking powder, and salt. Set aside.

In a large mixing bowl, whisk the sugar and melted butter until smooth. The mixture will be gritty.

Add the egg, sour cream, milk, and vanilla extract. Whisk until well combined.

Slowly add the dry ingredients to the wet ingredients, stirring just until incorporated. Do not overmix. The batter will be thick.

Divide batter evenly among the prepared liners, filling each about two-thirds full (about ¼ cup per cupcake).

Bake for 18 to 20 minutes, or until tops are lightly golden and a toothpick inserted in the center comes out clean.

Let cool in pan for 5 minutes, then transfer to a wire rack to cool completely before frosting.

Using a stand mixer with the whisk attachment (or a handheld mixer), beat butter at medium speed until smooth and creamy, about 2 to 3 minutes.

Add 3 cups powdered sugar, vanilla extract, milk, and salt. Beat on low until combined, then increase to high speed and beat for 3 minutes until light and fluffy.

If frosting is too thick, add more milk, a teaspoon at a time. If too thin, add more powdered sugar gradually.

Frost cooled cupcakes using an offset spatula or piping bag. Add sprinkles if desired.