Canning Jar Cakes 6 qt canning jars with lids spray oil baking pan large enough to hold jars cake mix of your choice or homemade Preheat oven according to directions. Spray insides of jars. Prepare cake batter according to directions. Fill jars to just under 1 cup mark. Bake 15 minutes and then test with knife or skewer. Carefully screw on tops, but not too tightly — the heat will create a vacuum and seal.