No one likes a chunk of crumbs in their frosting, which is why the crumb coat is an essential part of cake decorating. This thin layer of coating -- buttercream, ganache or glaze -- seals the sides of the cake, keeping it moist and trapping crumbs. Once applied you can cover the rest of the cake with a layer of fondant or a final coat of frosting, ganache or fruit. To crumb coat a square cake you need to take into account the sharp edges and use the right tools.
Use the Right Cover
Crumb coats can be made from frosting, ganache or a glaze, but buttercream is the most common type. Use a medium-consistency buttercream that spreads easily, but is still stiff enough to stand upright if you were to pull your spatula from it. Use a crumb coat that is a similar color to the top coat you're using. For example, if you're covering a cake with white fondant, you can use white buttercream.
Gather the Tools
A regular kitchen spatula won't work when crumb coating a square cake. Instead you need a small pastry bench-top scraper or angled cake spatula. If you use a regular cake spatula you'll pull the frosting from the edges of your square cake and not get a clean, crisp corner.
Try a Pastry Bag
Instead of applying frosting in dollops with your cake spatula, apply it with a pastry bag. Fill a pastry bag with a large decorating tip -- round or star tip, doesn't matter -- and gently pipe the frosting, ganache or glaze onto the sides of the cake and top, then piping a line over the corners of the square.
Do the Corners First
Before smoothing the rest of the cake, build your corners first. Start by holding your cake spatula vertically so the blade is flat against the cake and the edge of the blade is even with the corner of the square. Angle the cake spatula slightly and pull your frosting, ganache or glaze from the edge toward the middle of the square. Repeat on the other side of the corner until each side is even. Repeat the process on all four corners. Then, continue evening out your crumb coat on the top half and sides of the cake to finish.
Troubleshooting Tips and Tricks
Don't crumb coat your cake until it's fully cooled. Warm cakes soften your crumb coat and can make your square cake's corners crumble, or the frosting slide off. Try doing two thin crumb coats versus one thicker one. Two coats catch more crumbs and smooth the surface so it's ready for fondant or a final coat. If using multiple layers in a single tier, trim each layer evenly and line up the corners before crumb coating. Uneven layers or unaligned corners are difficult to crumb coat evenly and can tear as you spread the crumb coat on top.
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