To make your own buttermilk: pour 1 tablespoon of lemon juice or vinegar in a liquid measuring cup. Pour milk, any fat percentage, into the measuring cup until you have 1 cup. Let sit for 5 minutes before using.
If you don’t want to use coffee you can use hot water, but the flavor of the chocolate won’t be as pronounced. You can use decaffeinated coffee if you prefer.
The frosting recipe is written assuming you will be piping a generous amount of frosting on each cupcake. If you would prefer to simply frost with a knife then you can most likely halve the frosting recipe.