Preheat the oven to 350 degrees F. Line a 6-inch-round and an 8-inch-round cake pan with parchment paper; coat with cooking spray. Prepare the cake mix as directed; divide between the pans and bake 25 to 30 minutes. Let cool.
Level the tops of the cakes with a serrated knife (save the trimmings). Halve the cakes horizontally. Working on a cake board or plate, stack the 8-inch layers, then the 6-inch layers, frosting between each layer.
Hold a serrated knife at an angle and trim the edges of the layer cake all the way around to make an upside-down pot shape. Cover the cake with all but 1/2 cup of the remaining frosting; freeze 30 minutes.
Sprinkle 3 tablespoons cocoa powder on a sheet of parchment; put the fondant on top, make a small well in the middle and put the food coloring in the well. Knead until the color is even.
Dust a surface with cocoa powder; roll out the colored fondant into a 24-by-6 1/2-inch rectangle, trimming the edges to make them straight. Cut off a 24-by-1 1/4-inch strip of fondant; set aside.
Wrap the wide piece of fondant around the cake, pressing and smoothing it with your fingers. Trim the excess with a paring knife.
Reroll the fondant scraps and cut out a 5-inch circle; press the fondant circle on top of the cake to form the pot bottom. Invert the cake onto a board. Lightly brush the thin strip of fondant with water; attach to the top edge of the cake, allowing the strip to come slightly above the edge. Trim the ends.
Cover the top of the cake with the remaining 1/2 cup frosting, then crumble the cake trimmings on top.
When ready to serve, stick mint sprigs and edible flowers into the cake.
Photograph by Lisa Shin
Courtesy of Food Network Magazine