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Since the cake will be unwieldy once it is layered and iced, it's best to use the same plate on which you plan to serve the cake. Hold the knife parallel to the brim of the cake and slice about 1⁄4 inch (0.6 cm) off the top. Be careful to slice straight across, holding the knife level as you go.  Try to create a flat, even surface. This will serve as a base for the second layer. Remove the rounded top of the cake and discard it, or save it to eat. Don't attempt to cut off the top of the cake with a small knife, as this will result in an uneven surface.
Turn one cake onto a platter. Use a long serrated knife to trim off the rounded top of the cake. Finished.