In step 6. of the instructions, you need to use a skewer or metal cake tester. Insert the tester into the thickest part of the cake. I usually stick the tester in the very center and push down about halfway, take the tester out and if it comes out clean, the cake is done. If the tester comes out with wet batter stuck to it, then you know the middle of the cake isn't cooked, so leave the cake for a bit longer, at 5-minute intervals until the cake tester comes out clean.