Preheat the oven to 350°F. Line a 12-cup muffin tin with paper liners.

In a small mixing bowl, whisk to combine the flour, baking powder, salt, and baking soda.

In the bowl of a stand mixer fitted with a paddle attachment, combine the sugar and the butter. Beat on medium speed for about 3 minutes until light and fluffy.

Add the eggs and vanilla extract. Mix on medium-low until the eggs are combined. Add the sour cream and milk and mix until combined.

Gradually add the flour mixture and mix until the batter is smooth. Gently fold in the sprinkles.

Using a ¼-cup dry measuring cup, scoop the batter and level the top. Add this amount to one of the muffin liners. Repeat to fill the remaining liners. (Each liner should be about two-thirds full. You may have a small amount of batter left in the bowl, but it’s important not to overfill or the batter will spill over the edges when baking.)

Bake for 18 to 23 minutes, or until the tops are dry and the centers are springy to the touch. Transfer the cupcakes to a wire rack to cool.

Meanwhile, in the bowl of a stand mixer fitted with a paddle attachment (or a large bowl, using a handheld mixer), cream the butter and salt on medium speed for 2 minutes or until creamy.

Sift in the confectioner’s sugar, then drizzle in 3 tablespoons of cream and the vanilla. Beat on low speed for 1 minute, just until combined. If the frosting feels dry, mix in the additional tablespoon of heavy cream, 1 teaspoon at a time.

Increase the speed to medium and beat for 2 minutes until fluffy and creamy.

Transfer the buttercream to a piping bag fitted with a closed star tip—pipe about 3 to 4 tablespoons of icing on each cupcake. Decorate with more colorful sprinkles.