Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners. Set eggs, milk, and sour cream out to come to room temperature if needed.

In a medium bowl, whisk together flour, baking powder, baking soda, and salt. This keeps the crumb light and even.

In a large bowl, beat butter and sugar with a hand mixer (or stand mixer) on medium speed until light and fluffy, about 2–3 minutes. Scrape the bowl as needed.

Add eggs one at a time, mixing well after each. Stir in the vanilla. The batter should look creamy and smooth.

On low speed, mix in half the dry ingredients. Add the milk and sour cream, then the remaining dry ingredients. Mix just until combined. Do not overmix.

Gently fold in the sprinkles with a spatula. Use jimmies (long, thin sprinkles) to prevent bleeding.

Divide the batter evenly among the liners, filling each about 2/3 full. A standard cookie scoop helps keep them uniform.

Bake for 16–20 minutes, or until the tops spring back and a toothpick comes out clean or with a few moist crumbs. Rotate the pan once if your oven bakes unevenly.

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

Make the frosting: Beat the softened butter on medium-high until creamy, 1 minute. Add powdered sugar, vanilla, salt, and 1 tablespoon milk. Beat on low to combine, then on high for 2 minutes. Add more milk, a teaspoon at a time, until fluffy and spreadable.

Frost cooled cupcakes with a knife or piping bag. Top generously with colorful sprinkles. Serve and smile!