Overmixing or incorrectly mixing batter

You're making cupcakes incorrectly if you only use one bowl. When baking cupcakes from scratch, you should always use a hand mixer to combine the (room temperature) butter and sugar in a different bowl. The sugar is incorporated into the batter equally and contributes to fluffier cupcakes.

Your dry ingredients should be whipped together in a different bowl. Before adding the other liquid ingredients, properly incorporate each egg after you've creamed the butter and sugar. After that, combine your wet and dry components by folding them together. It's important to gently fold these ingredients together to maintain the batter's (and the cupcakes') fluffy texture. The delicate, airy texture you desire can be ruined by overmixing your batter. As a result, fold your wet and dry ingredients together until they are barely combined.