Preheat your oven to 350 degrees F. Line 24 muffin tin cups with paper liners and generously grease the liners with non-stick cooking spray.
In the bowl of a stand mixer (see note), whisk together the all-purpose flour, sugar, baking powder, and salt.
To the dry ingredients, add the milk, oil, eggs, and vanilla. Mix until well combined. (The cupcake batter will resemble pancake batter and be more on the liquidy side.) Use a rubber spatula to mix in the sprinkles.
Divide the cupcake batter evenly between the prepared muffin tin cups, about 1/4 cup each.
Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean. (The cupcakes may look wet and not ready, so be sure to do the toothpick test.)
Remove from the oven and transfer to a wire rack to cook completely before icing.
Frosting
Fit your stand mixer with the paddle attachment. Add the butter to the bowl and beat until pale and fluffy, about 1-2 minutes. Add the powdered sugar to the butter, ½ cup at a time, and mix until incorporated.
Add the vanilla and 1 tablespoon of heavy cream and mix. Add more heavy cream 1 tablespoon at a time until the frosting reaches your desired consistency.
Transfer the frosting to a piping bag fitted with a tip. Frost the cooled cupcakes and sprinkle with more rainbow jimmies.
Notes
Store any leftovers in the fridge in an airtight container for 3-5 days.
If you don't have a stand mixer, use a large mixing bowl with a hand mixer.
If the frosting is too thin, add more powdered sugar. If it is too thick, add more heavy cream.