Preheat oven to 375°F. Line two 12-cup cupcake cake pans with paper liners.
In a large mixing bowl, combine the water, cocoa powder, and espresso powder, if using, and allow it to sit for 5 minutes.
In a medium mixing bowl, whisk together the flour, sugar, baking soda, baking powder, and salt, set it aside.
Whisk in the milk, eggs, and oil into the cooled cocoa mixture. Stir in the flour mixture just until it starts to become smooth.
Fill the cupcake pan about ¾ full.
Bake for 20-25 minutes, or until the tops spring back when pressed down on.
Transfer to a cooling rack and allow them to cool completely.
Buttercream
In the bowl of your mixer with a paddle attachment, beat the butter on medium speed for 5 minutes until the butter has become light and fluffy.
Turn the speed down to low and slowly add in the powdered sugar and vanilla extract.
Add in the milk one tablespoon at a time. Turn the mixer back up to medium speed and mix for an additional 3 minutes.
Divide the buttercream into three separate bowls.
Add enough food coloring to each bowl to get a vibrant color, so you end up with one bowl of orange, one bowl of yellow, and one bowl of purple.
Assembly
Place each color of buttercream into a separate piping bag with a large star piping tip.
On one-third of the cupcakes, pipe a swirl of the yellow buttercream by placing the piping tip in the middle of the cupcake and adding pressure to the bag.
Swirl the bag around the cupcake, slowly making your way to the top.
Pipe a swirl of orange buttercream on another third of the cupcakes, then pipe two mini swirls on the sides of the top of the buttercream.
On the last third of the cupcakes, start by piping out a swirl of purple buttercream but keep piping at an angle while allowing the buttercream to begin to tip over.
Sprinkle the cupcakes with edible glitter.
Notes
Cocoa powder: I use unsweetened cocoa powder in this recipe.
Espresso powder: The espresso powder is optional.
All-purpose flour: To properly measure the flour, weigh it or sift/stir it to break it up. Lightly spoon into a measuring cup and then level. Avoid packing the flour.
Milk: Any type of milk from skim milk to whole milk will work. Using whole milk will give you softer, richer cupcakes. Nondairy milk, like almond milk, will also work.
Vegetable oil: Or a similar neutral oil.
Butter: I use salted butter. If using unsalted butter, add 1/4-1/2 teaspoon of salt to taste.