It can be difficult to bake cupcakes with perfectly domed tops. Baking soda or powder quality, correct measurements of ingredients and many other factors can cause flat or even deflated cupcakes. Unfortunately, while these cupcakes may still taste great, cupcakes without rounded tops may not be presentable to a discriminating audience. Although perfect cupcake tops require some trial and error, you can take a couple of precautions to increase the chances of your cupcakes turning out nicely rounded on top.
Check the age of your baking soda by stirring it, along with an acid such as vinegar, into very hot tap water. If your baking soda does not bubble, it is too old and will not cause your cakes to rise properly. Test baking powder in the same way but without the addition of an acid to ensure that it produces the same bubbling reaction.
Measure the precise amount of baking soda or baking powder recommended by your recipe into your cupcake batter. Too much baking soda or powder may cause your cupcakes to overexpand and then deflate, while too little can result in a flat, dense cupcake.
Mix your cupcake batter just enough so that all the ingredients are well-incorporated.
Fill each cupcake cup approximately three-quarters full with your cupcake batter. This is very important, as this step will cause your cupcakes to expand just enough to create a domed top instead of a flat one.
Open your oven only when your cupcakes are fully baked. Opening before they have finished baking may cause your cupcakes to deflate, even if they had produced domed tops during the baking process.
If you are using cake mix, baking powder or soda will already have been mixed into your batter for you, and you will not need to worry about measurements for these ingredients.
References and ResourcesThe Kitchn: Baking Tricks: How To Tell If Baking Soda Is Expired
Cupcake Bakeshop: Basics and Troubleshooting