First, increase the oven temperature to 425°F and start baking the muffins at this higher heat for the first 5 minutes. The blast of high heat helps create the initial rise and dome shape. Then, without opening the oven door, reduce the temperature to 350°F for the remaining 15-20 minutes of baking time. Secondly, fill the muffin cups almost completely full with batter, leaving just a tiny bit of space at the top for expansion. The extra batter will allow the muffins to puff up more dramatically as they bake.