My son wanted a Harry Potter cake for his birthday, and I thought a Homemade Harry Potter Cauldron Birthday Cake would be fun. Originally the cauldron was going to be top-half only, since I didn't think cake would support itself for a full cauldron. When I was thinking about what to use to make the lip of the cauldron, I though of rice krispie treats, since they're moldable, and decided that I could make some in the same bowl I baked the cake in to make the bottom half of the cake.
First I baked one chocolate cake mix in a Pam-sprayed medium sized metal bowl. It took close to 70 minutes to cook all the way through, after 45 minutes, I checked it with a toothpick every 10 minutes.
Next I made the rice krispie treats (3 tbsp butter melted, add 4 C mini marshmallows, then 6 C rice krispies). I pushed most of the rice krispie treat into the same bowl (the cake was now cooling on a wire rack), and kept out a little and using a plastic bag over my hand to prevent sticking, I molded a ring that would fit on top of the cake.
When the cake was cool, and the rice krispie treat was hard, I put the treat on a tinfoil covered piece of cardboard, and then put that on a cake stand that turns to make it easier to frost. I put a little frosting to hold it in place, then more frosting (2 C powdered sugar, 1.5 C shortening, and 1/2 C milk, and 1.5 tsp vanilla (I didn't use clear vanilla because I was going to tint the frosting anyway). Next I cut the cake so it had a flat side to put against the rice krispie treat (putting frosting between them to hold them together). Then I put the rice krispie treat ring on top, and frosted the whole thing. I used black Wilton coloring, and added a little green, and also some cocoa powder to get it as dark as I could. I made a little purple and green frosting to swirl inside the cauldron as the potion.