Hello! Baking times listed on boxed mixes are general guidelines, as every home oven can run slightly cooler or hotter than the set temperature. Fortunately, there are several reliable ways to determine if your sponge cake is properly baked all the way through.
First is the toothpick test. Insert a clean wooden toothpick, skewer, or thin knife directly into the center of the cake and pull it straight out. If it comes out clean or with a few dry crumbs, the interior is done. If it has wet batter sticking to it, the cake needs more time in the oven.
Second is the gentle touch test. Lightly press the center of the cake with your fingertip. A fully baked sponge cake should feel soft yet firm, and it should spring right back up immediately. If your finger leaves an indentation, the internal structure is not fully set.
Third is the visual indicator around the edges. When a sponge cake finishes baking, it will start to shrink just slightly away from the sides of the pan, and the top will have an even, golden-brown hue.
If the center is still wet while the top is already getting quite brown, you can loosely place a piece of aluminum foil over the top of the pan to prevent burning and continue baking in 3 to 5 minute increments until the center passes the toothpick and spring-back tests.