A moist sheet cake covered in puffy whipped cream icing almost melts in the mouth, unless poor storage causes the cake to become dry and the icing to become runny. Proper storage keeps the cake moist and the icing thick so it tastes just as good the second day as it did the first. Frosted cakes have a short shelf life, even with proper packaging and refrigeration.
Whole Cake Approach
A whole, frosted and uncut cake doesn't dry out as quickly as sliced cake. The whipped cream icing helps lock in the moisture so the cake doesn't require a tight wrapping to keep it fresh. Store the cake covered with an upturned bowl or in a cake carrier with a lid. Sheet cakes are often left in their baking pan, so only the top of the cake requires covering. If you must use plastic wrap, insert toothpicks into the top of the cake so the wrap doesn't rest against the icing and mess it up. Wrap the plastic over the top of the cake and beneath the cake board to keep the air out.
Any Way You Slice It
If you've already sliced the cake, the exposed sides may dry out quickly in storage. Wrap the cake tightly with plastic wrap. If you don't want to mess up the icing on top, add more icing to the exposed sides of the cake and package it as you would a whole cake. Or, wrap the lower exposed portion of each slice tightly with plastic wrap or foil, then use toothpicks to elevate the wrapping above the frosting.
Keep Your Cool
Whipped cream icing requires refrigeration to prevent spoilage. A warm room can also cause the icing to melt and become runny. Refrigeration helps keep the icing firm so it doesn't soak into the cake and make a soggy, unappetizing mess. Place the wrapped caked in the refrigerator away from strong-smelling foods, like onions, so the cake doesn't absorb any unwanted odors. You can store the refrigerated cake for up to two days.
Let Them Eat Cake
Cold cake may taste dry and flavorless, so it's best to bring it back to room temperature before serving. Place the wrapped cake on the counter no more than 30 minutes before you plan to serve it. Fortunately, whipped icing remains soft so you can fix any smudges in the icing with a clean spatula after you unwrap the cake. Keep the remaining cake covered after serving, and return it to the refrigerator promptly so the icing doesn't spoil or begin to run.