Brownies typically fall into one of two categories, cakey or fudgy. If you’re looking to perfect a fudgy brownie, there’s one technique that makes the biggest difference and it doesn’t involve changing the ingredients or ratios.
Rapping is an old-school baking technique that forces the air out of baked goods by lifting up the pan and dropping it during baking, causing the batter to deflate. For other baked goods such as cakes or muffins this would be the last thing you want. You’d lose all the structure for a cake. However for brownies, it removes tiny air pockets making it more dense, which gives a more fudgy texture.
You only need to lift up the pan about two inches then let it drop against the oven rack. You don’t need to repeatedly bang or smack the brownies. It’s best to do this when the brownies are about half-cooked. You’ll end up with perfectly fudgy brownies!
Source: the Kitchn