Let’s get baking! Here’s a detailed, step-by-step guide:

Cupcakes:

Step 1: Prepare Oven and Dry Ingredients

Arrange a rack in the center of your oven and preheat to 350°F (175°C). Line a standard 12-cup muffin tin with paper liners.

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

Step 2: Cream Butter and Sugar

In a large bowl, using a handheld mixer on medium-high speed (or in the large bowl of a stand mixer fitted with the whisk attachment), beat the softened butter and granulated sugar until light and fluffy. This will take about 3 to 4 minutes. It’s important to cream them well for a light and airy cupcake texture.

Step 3: Add Egg Whites and Vanilla

Add the egg whites and vanilla extract to the creamed butter and sugar. Beat until smooth and combined, about 2 minutes. Scrape down the sides of the bowl as needed.

Step 4: Incorporate Sour Cream

Add the sour cream and beat until just combined. Don’t overmix.

Step 5: Add Dry Ingredients and Milk

Add half of the dry ingredients to the wet ingredients and beat on low speed until just incorporated.

Add the milk and beat to combine.

Add the remaining dry ingredients and beat on low speed until just combined. Be careful not to overmix, as this can result in tough cupcakes.

Step 6: Bake the Cupcakes

Fill the cupcake liners about three-quarters full with batter. I like to use an ice cream scoop, to have similar portions.

Bake for 19 to 22 minutes, or until a toothpick or cake tester inserted into the center comes out clean. The tops of the cupcakes should be slightly golden.

Step 7: Cool the Cupcakes

Remove the cupcakes from the muffin tin and let them cool completely on a wire rack before frosting.

Frosting & Assembly:

Step 1: Make the Buttercream

In a large bowl, using a handheld mixer on medium-high speed (or in the large bowl of a stand mixer fitted with the paddle attachment), beat the softened butter until fluffy.

Gradually add the confectioners’ sugar, beating on low speed until combined.

Add 3 tablespoons of the heavy cream, the vanilla extract, and the salt. Beat on medium-high speed until the frosting is smooth and creamy, adding the remaining tablespoon of cream if needed to reach your desired consistency.

Step 2: Divide and Color Frosting

Transfer 1 cup of the white frosting to a piping bag fitted with a star tip. This will be for the “clouds.”

Add green food coloring to the remaining frosting and stir until your desired shade of green is reached. Start with a small amount of food coloring and add more as needed. Transfer this green frosting to another piping bag fitted with a large round tip.

Step 3: Frost and Decorate

Pipe a swirl of green frosting onto each cooled cupcake using the piping bag with the round tip.

Stand a piece of the rainbow candy upright in the green frosting on each cupcake.

Using the piping bag with the star tip and white frosting, pipe small “clouds” at the base of the rainbow candy.

Sprinkle gold sprinkles over the cupcakes for a finishing touch.