Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with cupcake liners.

In a large bowl, whisk together flour, sugar, baking powder, and salt.

Add the softened butter and mix on low speed until combined.

Add eggs one at a time, beating well after each addition. Stir in the vanilla extract.

Gradually add the milk, mixing until just combined. Do not overmix.

Gently fold in the rainbow sprinkles.

Divide the batter evenly among the prepared cupcake liners, filling each about two-thirds full.

Bake for 20-24 minutes, or until a toothpick inserted into the center of a cupcake comes out clean.

Let the cupcakes cool in the tin for 5-10 minutes before transferring them to a wire rack to cool completely.

In a large bowl, beat the softened butter until creamy.

Gradually add the powdered sugar, alternating with the heavy cream, beating until smooth and fluffy.

Stir in the vanilla extract and salt.

Once the cupcakes are completely cool, frost them with the vanilla buttercream. Decorate with extra sprinkles if desired.