I've not had this problem with the times that I have made them but instead find that they are very moist. A couple of thoughts: are your eggs at room temperature like the butter? Are you using whole milk and when you mix it alternate it with the milk. Since there are few ingredients in this recipe, it's important that they are good. I hope it works for you, because this is my "go to" for chocolate cake/cupcakes.
2 to 3 tablespoons of mayonnaise it works like a charm.
Try a box of instant chocolate pudding and cut the cocoa to 1/4 cup.