To make a basic fondant cake, make fondant using confectioners' sugar, and cover a baked and cooled cake with it. Make the baked cake using any cake mix, and follow the package instructions.
To make basic fondant, sift 2 pounds confectioners' sugar in a large bowl, and make a hole in the center. Mix 1/4 cup cold water with 1 tablespoon unflavored gelatin in a saucepan, and wait five minutes. Heat the mixture, and stir until the gelatin is dissolved and clear. Take off the heat, and add 1/2 cup white corn syrup and 1 1/2 tablespoons glycerine. Mix until well blended, and add 1 teaspoon of any flavoring.
Pour the mixture into the hole of the confectioners' sugar, and mix until the sugar is blended. Knead the icing with your hands until it is stiff. If the mixture is too sticky, add small amounts of confectioners' sugar. Knead the icing into a ball, and wrap it tightly in plastic wrap. Place the ball in an airtight container, and allow to rest at room temperature for eight hours.
Roll the fondant with a rolling pin on a clean surface dusted with cornstarch until it is 1/4-inch thick. Center the fondant on top of a cake iced with buttercream. Smooth the fondant using cornstarch until the surface is even and flat. Cut off excess icing, and decorate the cake.