1. Preheat the oven. Set the oven to 350 degrees F. Line a muffin pan with paper liners or grease the inside of each cup with cooking spray.
2. Cream the butter. In the bowl of a stand mixer fitted with the paddle attachment, or a large bowl using electric beaters, cream together the butter and sugar until flufAddfy. About 3 minutes on medium-high speed. Scrape down the sides of the bowl with a silicone spatula.
3. Mix the dry ingredients. In another large bowl, combine the flour, baking powder and salt. Using a whisk or fork, stir about 20 times to combine well. 
4. Mix the wet ingredients. Pour the milk into a glass measuring cup with a spout. Add the vanilla and stir well. 
5. Add the eggs. Crack 1 egg into a small bowl. Add the egg to the batter and mix on medium speed for 1 minute. Scrape down the sides of the bowl and repeat with the second egg. 
6. Add the dry ingredients to the wet ingredients. Pour half the flour mixture into the butter mixture. Mix on low speed for 10 seconds. Add half the milk mixture and beat for 10 seconds. Stop the mixer and scrape down the sides. Repeat until both mixtures have been completely added to the large bowl and mixed to incorporate. 
7. Color the batter. Evenly divide the batter among 3 small bowls. Add 2 or 3 drops of food coloring to each bowl and stir until the colors are uniform in each bowl. 
8. Fill the pan. Use a spoon to place 1 scoop of blue batter into each prepared muffin pan. Use a second spoon to add a scoop of red batter, and a third to finish with a scoop of green batter. Each cup should be about 3/4 full. 
9. Bake. Carefully place the pan onto the center rack of the preheated oven and bake for 18-20 minutes. 
10. Let cool. Remove the pan from the oven and allow to cool for at least an hour before decorating.

TO DECORATE

1. Color the frosting. Add 1-2 drops of blue food coloring into the frosting and stir until the color looks like sky blue. 
2. Frost cupcakes. Once the cupcakes are completely cool, spread an even layer of blue frosting on top. 
3. Cut the rainbows. Using kitchen shears (or a sharp knife and cutting board), cut the rainbow candy strips into 4-inch strips. 
4. Cut the marshmallows. Using kitchen shears again (or a sharp knife and cutting board), cut mini marshmallows into 4 pieces. Roll each piece in your palm until they’re round. Like clouds. 
5. Assemble the rainbows. Push one end of the rainbow strip into the frosting on one side of the cupcake, and then push the other end into the other side. Place 3-4 “clouds” at the base of each side of the rainbow.