Knead Gum-Tex into about 20 oz. black fondant; roll out to 1/8 in. thickness. Make fondant ropes of different lengths and diameter in a variety of colors. Arrange over black fondant; roll over logs until both layers are 1/8 in. thick. Using patterns cut one of each size triangle (4 in x 7 in., 6 in. x 7 in., and 8 in. x 7 in.) Reverse pattern and cut a second of each size triangle. Set on cornstarch-dusted cake board; dry at least 48 hours.
Bake and cool 2-layer 6 in. and 8 in. square cakes. Stack cooled cake layers on cake boards. Prepare for fondant by lightly icing with buttercream.
Roll out remaining black fondant 1/8 in. thick. Cover cake; smooth. Prepare for stacked construction. Stack cakes.
Tint melted cocoa Candy Melts candy black. Make topper by attaching 4 in. x 7 in. triangles together at a 90° angle at the 7 in. edge with melted candy. Let set. Attach remaining triangles to cake sides using tinted melted candy. Pipe black candy at 7-in. seam, if desired. Position topper on cake.