Preheat the oven to 350 degrees. Prep your muffin tin(s) with liners and set aside.

In the bowl of a stand mixer, combine the softened butter and sugar. Cream until smooth.

Add in the egg whites, oil and vanilla. Mix well.

Add the milk and mix to combine. The mixture may appear curdled - this is normal.

Add in the salt, baking powder and flour and mix well. Stop the machine, scrape down the sides and mix again.

Measure out the batter and fill each cavity until about ½-⅔ full. Bake in the preheated oven for approximately 14 minutes or until the cupcakes are golden brown and the center is set. Allow the cupcakes to cool completely before decorating.

In the bowl of a stand mixer, combine the butter, vanilla, salt and about half of the powdered sugar.

Mix on low speed until combined. Add in the remaining powdered sugar and the milk.

Mix on low speed until combined. Once combined, bump up the speed to medium for an additional minute until the frosting is fluffy and smooth.

Divide the frosting - measure out ½ of a cup for each of the colors. The remaining frosting will stay white and it can be transferred to a piping bag. Tint each of the colors and transfer that frosting to each individual bag.

Line out a strip of plastic wrap approximately 18-24 inches long. Pipe each color onto the plastic wrap in rainbow order - purple, blue, green, yellow, orange and then pink.

Roll the plastic wrap and coloring frosting so that they overlap. Place the log of frosting into a piping bag fitted with a large star tip.

Use the colored frosting bag to create an inverted “U” or horseshoe shape. Twist the piping bag mid squeeze to show all the colors and add dimension.

Immediately place the mini marshmallow “clouds” before the frosting crusts over.