There's no magic potion in this cauldron-it's a chocolate cake! To create the edible pot, we trimmed a four-layer cake, leaving a wide lip around the edge. We frosted the whole thing black, then added doughnut handles and gummy-candy flames. That bubbling green stuff on top is just lime gelatin.
Preheat the oven to 350 degrees F. Coat four 9-inch cake pans with cooking spray. Prepare the cake mixes; divide the batter among the pans and bake and cool as directed. Meanwhile, prepare the gelatin as directed.
Level the domed tops of the cakes with a long serrated knife. Stack 3 of the cakes on a small cardboard circle with a thin layer of chocolate frosting between each cake. Freeze at least 30 minutes.
Put the 3-layer cake on an upside-down cake pan (so trimming will be easier). Using a small serrated knife, trim the bottom edge of the cake to round it out, then round out the top edge. Frost the top.
Set the remaining cake layer on top of the 3-layer cake; freeze at least 30 minutes. Trim around this layer at an angle, cutting inward so it resembles a shallow bowl.
Cut and scoop out the center of the top cake layer, about 6 inches wide and 2 inches deep. Cover the inside of the hole with the white frosting.
Dye the remaining chocolate frosting black. Cover the exposed cake with a thin layer of black frosting; freeze about 20 minutes, then cover with the remaining frosting (reserve about 1 tablespoon). Return the cake to the freezer.
Once the gelatin is firm, stir until clumpy. Return to the refrigerator until ready to serve.
Make the flames: Using a paring knife, cut the gummy fruit slices in half to make 2 thinner slices. Press a few pieces of different colors together with your fingers as shown. Repeat to make about 20 flames.
Press the flames around the bottom of the cake. Cut off one-third of each doughnut. Using the reserved black frosting as glue, press the doughnuts into the sides of the cake for handles. Add the gelatin just before serving.