A bubbly brownie is not an uncommon sight for bakers; luckily, those air bubbles have no effect on the quality or taste of your finished product. If you'd still rather not see those little craters, a few easy tweaks to your tools and techniques can help you eliminate most of the air pockets that result in bubbles appearing on the surface of your baked brownies.
A bubbly layer on top of a pan of brownies usually indicates that too much air was incorporated into the batter during the mixing process. In general, over-mixing and mixing with a whisk introduces more air into the batter than mixing until just combined or using a spoon. If your brownie recipe calls for a specific utensil or cautions you not to over-mix, this is usually to achieve a denser and more tender result with fewer air bubbles.
Too Much Leavener
Because chemical leaveners produce air bubbles themselves, brownie recipes often leave out a chemical leavener such as baking soda or baking powder altogether to achieve a fudge-like, bubble-free brownie. If your recipe calls for a chemical leavener, measure carefully to avoid adding too much, and whisk or sift the leavener into the dry ingredients to distribute it evenly before combining with the wet ingredients.
Tapping Out Bubbles
A commonly used technique to reduce the amount of air in your brownie batter, therefore avoiding bubbles from forming atop your finished product, is to tap the pan of uncooked batter on your countertop before placing it in the oven. This helps trapped air work its way to the surface before the batter sets during baking. To further reduce the chances of a bubbly brownie top, carefully repeat this step about halfway through the cooking process to release any additional air bubbles that have formed.
Bubbles vs. Crust
Some recipes and techniques are actually intended to result in a crunchy, almost separate top layer on your brownies. These meringue-like crusts, not to be mistaken for air bubbles, are the result of a thorough beating of the butter, sugar and eggs before the dry ingredients are added. If you would rather not end up with a crust, simply mix the wet ingredients as little as possible; in either case, do not over-mix once the dry ingredients are added.