Peeking in on your cake while it's baking is just part of the process. You need to make sure it's not baking too fast and that everything is going according to the recipe. While a few air bubbles popping up to the surface is normal, excess air bubbles is a sign the cake might not bake properly. Once it's in the oven, however, there isn't much you can do to reverse the bubbles.
Too Much Creaming
Creaming is the process where fats -- typically butter -- and sugars are mixed together until light and fluffy. This process is crucial in cakes, because it provides the necessary volume and air for a fluffy crumb. Starting out at too high of a speed or creaming for too long, however, creates excess bubbles. These bubbles rise to the surface while baking and, once done, leave you with a cake that has holes and tunnels under the crust. Avoid this by only creaming ingredients per the recipe instructions. Follow the amount of time you should cream and the speed recommendations.
Too Much Leavener
Leaveners, such as baking soda or powder, heighten the bubbles created during the creaming stage of a recipe. They do so by allowing the bubbles to form and rise quickly through the batter -- raising the cake during the baking process. If a cake bubbles too much, you might want to reduce the amount of leavener. Try reducing by 1/8 teaspoon at first.
Overmixed Dry Ingredients
Cake mixes have dry ingredients added to wet ingredients. Don't overmix the batter when you combine the two. Flour contains gluten, which is a strong, stretchy protein that forms when you mix flour with wet ingredients. It provides structure to your cake, but when you overmix the flour you'll encourage too much gluten to form in the batter. Excess gluten creates problematic air bubbles in your batter that not only leave you with a holey cake, but makes your cake rubbery, tough and dense -- similar to bread. Also, pre-sift your dry ingredients together. Doing so limits how much you'll have to mix once they're added to the wet ingredients.
Fixing Your Mistakes After the Fact
Once a cake is in the oven, you cannot correct bubble issue. If you have already baked a cake that has bubbles, you still have a few options. As long as the cake isn't dry, fallen or crumbling, fill in the holes on the exterior surface with buttercream before adding a final coat to the cake. Or, crumble the cake into pieces and mix it with buttercream. Roll the mixture into balls and dip them in melted chocolate to create cake pops.
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