Begin by adding the flour to a bowl with the baking powder, salt, and sugar. Stir well to combine with a spoon or whisk.
Stirring in between each ingredient, add the milk, oil, and eggs to the flour mixture.
Add in the vanilla bean paste. Stir for 1-2 minutes to fully incorporate the paste throughout the batter.
Preheat the oven to 350F and place cupcake liners in a muffin pan. Using a large ice cream scoop filled ¾ of the way full, scoop the batter up and pour into the cupcake liner. Repeat with other cupcakes.
Bake for 20 minutes at 350F. Allow to cool completely.
While the cupcakes are cooling, prepare the buttercream frosting. Add two sticks of softened butter into a large bowl. Sift four cups of powdered sugar on top of the butter. Using a hand mixer, start on a low speed to combine the butter and powdered sugar. A low speed ensures powdered sugar does not fly out of the bowl.
Increase the mixer to a medium speed and add the heavy whipping cream 1 tablespoon at a time. Mix for 4-5 minutes until thickened. Then, add the food coloring and mix enough to combine, 1-2 minutes at a low speed.
Add the buttercream to a piping bag with a star tip (optional).
When the cupcakes have cooled off, cut two slits in the bread on opposite sides. These slits will serve to help your rainbow stand up on the cupcake. Place each edge in the slit to create an arch/rainbow.
Pipe the buttercream under the rainbow first and then around the edges of the cupcake.
Add the sugar pearls to the edges of the rainbow for the ‘clouds’.