Prepare the Oven: Preheat the oven to 350°F and line a 12-cup muffin tin with paper liners.
Combine Dry Ingredients: In a small mixing bowl, whisk together the flour, baking powder, salt, and baking soda until evenly combined.
Cream Butter Sugar: Add the softened butter and granulated sugar to the bowl of a stand mixer fitted with a paddle attachment. Beat on medium speed for about 3 minutes, until the mixture is light and fluffy.
Add Eggs Vanilla: Add the eggs and vanilla extract to the butter mixture. Mix on medium-low speed until the eggs are incorporated.
Add Dairy: Add the room-temperature sour cream and whole milk, then mix until the ingredients are combined.
Finish the Batter: Gradually add the dry ingredient mixture to the wet ingredients, mixing just until the batter becomes smooth. Avoid overmixing once the flour has been incorporated.
Fold in Sprinkles: Gently fold the rainbow jimmies into the finished batter, distributing them evenly without overmixing.
Portion the Batter: Use a ¼-cup dry measuring cup or a 2-ounce spring-loaded scoop to portion the batter into the prepared liners. Level the scoop before adding each portion. Fill each liner about two-thirds full, taking care not to overfill them; a small amount of batter may remain. Bake the Cupcakes: Bake at 350°F for 18 to 23 minutes, or until the tops are dry and the centers feel springy when gently touched.
Cool the Cupcakes: Transfer the baked cupcakes to a wire rack and allow them to cool.
Cream the Butter: While the cupcakes cool, place the room-temperature butter and salt in the stand mixer fitted with a paddle attachment, or use a large bowl with a handheld mixer. Beat on medium speed for 2 minutes, or until creamy.
Add Sugar Cream: Sift in the confectioners’ sugar, then add 3 tablespoons of heavy cream and the vanilla extract. Beat on low speed for 1 minute, just until the ingredients are combined.
Adjust Frosting: If the buttercream seems dry, add the remaining tablespoon of heavy cream 1 teaspoon at a time, mixing as needed until the frosting reaches a creamy consistency.
Whip the Buttercream: Increase the mixer speed to medium and beat for 2 minutes, until the buttercream is fluffy and creamy.
Pipe the Frosting: Transfer the buttercream to a piping bag fitted with a large closed star tip. Pipe approximately 3 to 4 tablespoons of buttercream onto each cooled cupcake.
Add Sprinkles: Finish the frosted cupcakes with additional colorful jimmies or sprinkles for decoration.
