• Place cupcake liners into the cupcake tins and preheat the oven to 350 degrees Fahrenheit.
• Add .5 cup butter and 1 ⅔ cup sugar into the mixing bowl. Cream on medium high speed for 2 - 3 minutes.
• Add in 6 egg whites, ¼ cup oil and 1 teaspoon of vanilla and mix well.
• Add in 1 ¼ cup of milk and mix until combined. It may appear curdled, this is normal.
• Add in ½ teaspoon of salt, 2 ½ teaspoon baking powder and 2 cups of flour. Mix on medium low speed, scraping sides down as needed until well combined.
• Fill cupcake liners about ⅔ of the way full and bake for about 14 minutes or until golden brown.

For rainbow frosting

• Combine 1 cup butter, 1 teaspoon vanilla, 1 teaspoon salt and 2 cups powdered sugar in mixing bowl. Mix on low until well combined.
• Add 3 tablespoons milk and remaining 2 cups of powdered sugar. Mix on low until well combined and then bump up to medium and mix an additional two or so minutes until fluffy and smooth.
• Divide frosting evenly into 6 bowls. One will stay white, the others will get a few drops of the food coloring. Mix until well combined.
• Add each color to a piping bag.
• Lay out a strip of plastic wrap about 18 - 24 inches long. Pipe each color of frosting onto the plastic wrap in rainbow order - purple, blue, green, yellow, orange, pink.
• Roll the plastic wrap around the frosting so they overlap and then place the log into a piping bag fitted with a large star tip.
• Once cupcakes are cooled, pipe a mound of white frosting on each. Use a knife of spatula to flatten it.
• Then pipe the colored frosting on each in a U or rainbow shape.
• Add mini marshmallows on the ends of the "rainbow" as clouds.