Things You'll Need
Non-textured paper towel
2 fondant smoothers
Fondant-covered cube cakes can be used in a variety of ways, from baby blocks for a baby shower to dice for a casino-themed party. The trick to making these cube cakes work is eliminating all wrinkles and air bubbles in the fondant. Buttercream frosting is usually used to hold fondant to a cake, but buttercream edges are usually slightly rounded, despite a cake cut into a perfect cube. Fondant smoothing tools allow you to sharpen the rounded edges. You can use packaged fondant from the store or make your own marshmallow fondant.
Smooth out the frosting on the cake to make the edges as straight as possible. Eliminate any ridges or bubbles which can show through the fondant. Smooth out buttercream frosting with an icing spatula and rub it with a non-textured paper towel to finish after the frosting forms a crust.
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Spray the frosting with a very fine mist of water, using just enough to make the surface tacky so it acts as a glue to secure the fondant to the cake.
Roll the fondant out to about 1/4-inch thick; thicker fondant won't show the crisp edges as well.
Drape the fondant over the cake cube, removing as much of the wrinkles as possible before setting it down on the cake top. The sides of the fondant should drape away from the cake sides for now.
Rub the top with a fondant smoother tool to press it flat and push out any air bubbles. If any bubbles become trapped under the fondant, lift the draped edge closest to the air bubble and push the air out with the fondant smoother.
Hold a corner of the fondant out from the corresponding corner of the cube-shaped cake. Press the fondant against the side of the cake and against the corner with your hand. Rub your hand along the fondant to smooth it out, starting from the top edge of the cake and the corners and slowly working your way down to the bottom of the cake. Smooth it the rest of the way with the fondant smoother. Lift the bottom of the fondant flap as needed to push out any air bubbles. Repeat this process with the remaining four sides of the cube; the bottom side doesn't get covered with fondant.
Glide the fondant smoother along the top edge and corners of the cake to sharpen the edges as much as possible. For example, to make the top edge sharp, press one fondant smoother against one side of the cube, with about half of the smoother extended above the cake edge. Place a second smoother on top of the cake, butted directly against the exposed portion of the first smoother. Press on the fondant slightly with each smoother while pushing the smoothers together. Glide them back and form along the edge as a single unit to sharpen the edge. Repeat with the remaining three top edges and each of the four corners.
Cut away the overhanging fondant around the base of the the cube, using a sharp knife to make a clean cut. Press the ends of the fondant tight against the cake, using the fondant smoother. Push any remaining overhanging fondant under the cake.
If you want perfect 90-degree angle edges, cover the cube cake with chocolate ganache and shape it with bench scrapers used as straight edges. Set bench scrapers so the bottom edge rests on the cake board and scrape it around the outside to remove any excess ganache, if applicable. Hold one scraper against a corner or edge of the cube and pull the other scraper tightly against it to make the edges perfectly square. This is the same basic method used to sharpen the corners of fondant with the fondant smoothers.