Preheat the oven to 375°F. Line two cupcake pans with paper cupcake wrappers.

In a small bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt. Set aside.

In a large bowl, beat the butter and sugar together until the mixture is pale yellow and fluffy, about 3-4 minutes. Add the eggs one at a time until incorporated. Then, mix in the vanilla extract and sour cream. Scrape down the sides of the bowl.

Stir in half of the flour mixture followed by half of the milk. Repeat with the remaining flour mix and milk, stirring just until combined. Don't overmix!

Divide the batter evenly between the 24 cupcake wrappers, until the cups are about 2/3 full each. Bake the cupcakes for 12-14 minutes, just until an inserted toothpick has a few moist crumbs.

Let the cupcakes cool in the pan for a few minutes. Transfer the cupcakes to a cooling rack and allow them to cool completely before frosting.

In a large bowl, beat the butter for a couple of minutes until it is fluffy. Add the cocoa powder and mix until combined.

Next, add the powdered sugar 1 cup at a time until the frosting comes together. Then, add the whole milk or heavy cream 1 Tablespoon at a time until the frosting is smooth and reaches your desired texture.

Whip the buttercream for 5-6 minutes until it is light and fluffy. Pipe the buttercream onto the cooled cupcakes with a piping bag and tip. Or, use a butter knife or offset spatula to spread the frosting onto the cupcakes. Decorate as desired. Enjoy!