Preheat your oven to 350°F (175°C) and line a cupcake pan with liners.
In a medium bowl, whisk together the all-purpose flour, baking powder, and Kosher salt.
In a large mixing bowl, cream together the room temperature unsalted butter and granulated sugar until light and fluffy, about 3-5 minutes.
Add the eggs one at a time, mixing well after each addition, then mix in the vanilla extract.
Gradually add the dry mixture to the wet mixture, alternating with the whole milk, mixing just until combined.
Gently fold in the rainbow sprinkles using a spatula.
Scoop the batter into the cupcake liners, filling each about two-thirds full.
Bake for 15-18 minutes, or until a toothpick inserted comes out clean.
Cool the cupcakes in the pan for about 5 minutes, then transfer to a wire rack to cool completely.
For the frosting, beat the room temperature unsalted butter until smooth, then gradually mix in the powdered sugar, milk, and vanilla extract until creamy.
Once the cupcakes are cool, frost each generously and top with additional rainbow sprinkles.
