Prepare the Cupcakes:
Preheat your oven to 350°F (175°C). Line a muffin tin with cupcake liners.
In a medium bowl, whisk together the flour, baking powder, and salt.
In a large bowl, beat together the softened butter and sugar until light and fluffy, about 2-3 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, alternating with the milk. Start and end with the dry ingredients. Mix until just combined.
Gently fold in the rainbow sprinkles, being careful not to overmix (so the colors don't run).
Divide the batter evenly among the cupcake liners, filling each about 2/3 full.
Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean.
Let the cupcakes cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.
Prepare the Vanilla Frosting:
In a large bowl, beat the softened butter until creamy.
Gradually add the powdered sugar, one cup at a time, beating well after each addition.
Add the vanilla extract and 1 tablespoon of milk, and beat until the frosting is smooth and fluffy. Add more milk if needed to reach your desired consistency.
Once the cupcakes are completely cool, frost them using a piping bag or spatula.
Optional: Sprinkle additional rainbow sprinkles on top for an extra festive touch.