Prepare the 9 x 12-inch Pan and Preheat the Oven - Preheat the oven to 350 degrees Farenheit. Spray the 9 x 12-inch baking tray with cooking spray.
Combine the Dry Ingredients - In a large mixing bowl, combine the gelatin, granulated sugar, flour, salt, baking powder, and baking soda.
Add the Wet Ingredients - To the dry ingredients, add the oil, orange juice, lemon juice, vanilla, and eggs. Mix until the key lime batter is well combined.
Bake the Cake - Carefully pour the cake batter into your 9 x 12-inch baking tray. Bake the cake for 35-40, using a toothpick to check for doneness. If the toothpick comes out clean, your key lime cake is done.
Cool the cake in the pan for nearly five minutes. Then, allow the key lime cake to cool on a wire rack.
Make the Glaze - In a small bowl, mix together the lime juice and confectioners' sugar using a fork. Pour over the cake while it is still warm.
Make the Frosting - In a large bowl, cream together the butter and cream cheese using an electric mixer. Slowly add in the confectioners' sugar and beat until the frosting is well combined.
Decorate the Cake - Once the cake is fully cool, spread the cream cheese frosting over the top and sides of the cake. Slice and serve your cake.