The key to any good cake is to beat plenty of air into it. You know that part of a recipe that says "Beat on high for 2 minutes..." or whatever? You have to do that.
It rises higher and lighter. You're also better off using a cake flour for cupcakes so they're lighter and fluffier.
The secret to good buttercream is a pinch of salt. It cuts that sickly sweet taste to a mound of frosting.