One-Bowl Chocolate Cupcakes with Buttercream Frosting

1 1/4 cups Dutch-process cocoa powder

2 1/2 cups sugar

2 1/2 teaspoons baking soda

1 1/4 teaspoons baking powder

1 1/4 teaspoons salt

2 large whole eggs, plus

1 large egg yolk

1 1/4 cups milk

1/2 cup plus 2 tablespoons vegetable oil

1 1/4 teaspoons pure vanilla extract

1 1/4 cups warm water BUTTERCREAM FROSTING:

4 large egg whites

1 1/4 cup sugar

3 sticks unsalted butter, room temperature, cut into tablespoons

1 teaspoon pure vanilla extract