Be sure to measure your flour correctly by scooping the flour into the measuring cup with a spoon and then level off with a knife. Make sure your large eggs are room temperature. Place your eggs in a bowl of warm water for 10 minutes before using them in your recipe Double check that your baking powder is still fresh. Because this recipe uses melted butter there is no creaming step to incorporate air, so you need to make sure the baking powder is fresh so your cupcakes rise properly. To check the freshness, add a teaspoon of baking powder to some hot water. If it bubbles, it's still fresh. If it doesn't throw it out. Use an oven thermometer to make sure that your oven is heated correctly.