Larger-sized cakes are great for serving at special events, but these non-standard sizes will require you to adjust the amount of boxed cake mix you use to produce the desired amount of cake. Additionally, larger cakes require a longer baking time at a lower temperature to ensure the cake cooks completely and does not burn or dry out. When you use boxed cake mix to make a large cake, you will double both the mix as well as the amounts of all ingredients listed in the directions on the box and adjust the baking time accordingly. This little bit of extra effort will be well worth it when you serve the cake and all your guests can enjoy it.
Preheat the oven to 325 degrees F.
Crack and lightly beat the number of eggs required in a large measuring cup. Add the number of eggs listed in the directions on both boxes. For example, if one box of cake mix requires three eggs and the other mix also requires three eggs, you'll be using six eggs when you double your mixes.
Open and pour both dry mixes into your mixing bowl. Use a spatula or a metal whisk to gently break up any lumps in the mix.
Add the eggs and oil to the dry ingredients. As with the eggs, add together the amounts of oil called for on each box of mix.
Mix the batter with a stand or handheld mixer on low speed for two to three minutes, then increase the speed to medium high for another five to six minutes. Halfway through mixing the batter, scrape down the sides and bottom of the bowl with the spatula.
Grease your baking pan(s) with butter and dust them with flour. For the cleanest removal, cut a piece of parchment paper to line the bottom of your pan(s).
Pour the cake batter into the pan(s) and bake for approximately 40 to 45 minutes. Different shapes and sizes of cake pans will require different baking times, but typically, the minimum amount of baking time will be 40 minutes. After the required baking time, test the cake for doneness by inserting a toothpick into the center of the cake. If the toothpick comes out clean, the cake is done. If batter or numerous crumbs cling to the toothpick, continue baking the cake in 5-minute increments and checking for doneness each time until the cake is cooked completely.