Preheat oven to 350°F. Spray a 9x13 baking pan with nonstick cooking spray and set aside.
Whisk together all of the dry ingredients in the bowl of a standing mixer: flour, cocoa powder, sugar, baking soda, baking powder, and salt.
Add the wet ingredients into the bowl with the dry ingredients: milk, eggs, vanilla, and oil.
Beat together with a hand mixer or whisk. Gradually add in the boiling water while mixing to help break apart the batter.
Pour the batter into the prepared muffin pan.
Bake for 35 minutes or until a toothpick inserted into the cakes comes out with just a few crumbs.
Cool cupcakes completely.
Meanwhile, prepare the frosting.
In the bowl of a standing mixer or with a hand mixer, slowly begin to beat together softened butter, vanilla, and cocoa powder.
Rotate between adding the powdered sugar and heavy cream until both are fully incorporated and the frosting is fluffy and smooth.
Pipe the frosting over the cooled cupcakes.
Serve.
Notes
Make sure that all the refrigerated ingredients used for making the cupcake batter are at room temperature before mixing all the ingredients together. This helps the batter to mix evenly and smoothly.
These cupcakes are made with milk instead of buttermilk which gives them a light and fluffy texture. You could use buttermilk instead should you want a more dense cupcake.
Always use a shiny cupcake/muffin pan instead of a dark one as dark pans will absorb more heat from the oven and you run the risk of having your cupcakes browning on the edges and top while still being undercooked on the inside.