1. Preheat the oven to 350°F and grease a 9×13-inch baking pan. Line with parchment paper if you want easier lifting and cleaner slices.

2. In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

3. In a large mixing bowl, beat the softened butter and granulated sugar until pale and fluffy, about 3 to 4 minutes.

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

5. Add the dry ingredients in three additions, alternating with the milk, beginning and ending with the dry ingredients. Mix just until combined.

6. Gently fold in the rainbow jimmies sprinkles with a spatula.

7. Spread the batter evenly into the prepared pan. Bake for 32 to 38 minutes, or until a toothpick inserted in the center comes out clean.

8. Let the cake cool completely in the pan before frosting.

9. To make the frosting, beat the butter until creamy. Add the powdered sugar, vanilla extract, salt, and 3 tablespoons of heavy cream or milk. Beat until fluffy, adding the last tablespoon of cream if needed for a smooth spreadable texture.

10. Spread the frosting over the cooled cake and top with extra sprinkles. Slice and serve.