1. Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with cupcake liners.
2. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
3. In a separate large bowl, cream the butter and sugar until light and fluffy. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
4. Gradually add the dry ingredients to the wet mixture, alternating with the milk, starting and ending with the flour mixture. Mix until just combined.
5. Gently fold in the rainbow sprinkles.
6. Divide the batter evenly among the cupcake liners, filling each about 2/3 full.
7. Bake for 18–20 minutes, or until a toothpick inserted into the center comes out clean. Let the cupcakes cool in the pan for 5 minutes before transferring to a wire rack to cool completely.

For the Vanilla Buttercream:

1. In a large bowl, beat the softened butter until smooth. Gradually add powdered sugar and beat until light and fluffy.
2. Add vanilla extract and 2 tablespoons of cream, beating until the frosting reaches a spreadable consistency. Add more cream if needed.
3. Once cupcakes are completely cooled, frost with vanilla buttercream and add additional sprinkles for decoration.