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.