Preheat oven to 350°F (177°C).

In a stand mixer, cream butter and sugar together. Add egg, sour cream, and vanilla.

In a separate medium bowl, whisk together the flour, baking soda, salt and baking powder.

Alternate between adding dry ingredients to wet ingredients in stand mixer and whisking in milk slowly. On your last addition, add in sprinkles.

Whisk until batter is smooth.

Pour batter into lined muffin tin, filling cupcake liners about halfway full.

Bake for 15-18 minutes until cupcake centers bounce back to the touch. Insert a toothpick to test — if it comes out clean or with a few crumbs, they are ready. If there is batter on the toothpick, they need a little more time.

Cool cupcakes on a baking rack completely before frosting with buttercream icing.

Use a hand mixer to cream butter and vanilla together.

Add ½ cup of powdered sugar.

Continue adding powdered sugar ½ cup at a time until icing is at desired consistency.

Add heavy cream. Icing should be smooth but hold its shape fairly well.

Pipe icing onto room temperature cupcakes. Top with extra sprinkles.