Most cakes have a tendency to rise more in the middle than at the edges, a characteristic called "doming." For a cake made of a single layer, especially if it will not be covered with icing, some slight doming is perfectly acceptable and may even be attractive. Usually, though, doming is considered a flaw. Doming is caused by the edges of a cake baking and setting while the middle is still rising, and there are various techniques to address it.
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Controlling the Baking
Check the temperature of your oven often with an oven thermometer. Many home ovens consistently bake at temperatures higher than their thermostat says, and this contributes to doming. Move the thermometer to different spots in your oven to learn if it has notable hot and cool spots.
Space cake pans evenly during baking. When 2 cakes are close to each other, they will tend to rise on the sides that face each other. Cakes baked too near the oven's walls will rise more on the side facing the oven and less on the side facing the wall.
Wrap your metal pans with the cake-baking strips available in many department stores, or with strips of fabric soaked in water. This limits the ability of the pan's sides to transmit heat and prevents the cake from baking too quickly at the edges. Switching from dark- to light-colored pans will also help minimize doming from this cause.
Use silicon pans to minimize doming. Silicon pans do not conduct heat the way metal pans do, and will not bake the edges as quickly. Silicon pans are harder to handle and must be rested on a sheet pan for baking, but cakes do not dome or stick. Silicon pans inhibit crust browning and formation, which may or may not be desirable.
Controlling the Batter
Reduce the amount of gluten in your cake batter by replacing all-purpose flour with cake flour. Gluten in the batter allows the cake to stretch and continue rising as the leavening ingredients release bubbles into the hot batter. Less gluten means less doming. If cake flour is unavailable, pastry flour will also improve the batter, though to a lesser degree.
Replace 1 tbsp. of all-purpose flour with 1 tbsp. of cornstarch, for every cup of flour in the batter. This is a standard substitution for cake flour and will have a similar effect.
Reduce the strength of gluten in a cake batter by adding extra butter. The fat lubricates the gluten strands and prevents them from bonding as strongly as they otherwise might. Acidity has the same effect, so adding lemon juice or other acidic ingredients where appropriate will also minimize doming.
Increase the leavening slightly in your batter to encourage the edges of the cake to rise more quickly as they bake. As little as 1/4 tsp. of baking powder can make a difference, or 1/16 tsp. of baking soda.