Fresh, juicy strawberries and other fruit make a beautiful filling for cakes, but they're extremely perishable and can sharply limit your options for advance preparation. They require refrigeration, which can be problematic if you plan to cover the cake with fondant. In humid climates your fondant often "sweats" when it comes out of refrigeration, becoming sticky and losing the crisp appearance of its decorations. You can put berries in your fondant-covered cake, as long as you keep the fondant cool and dry throughout the process.
Assembling the Cake
Strawberries exude a lot of juice, especially if they're sweetened, so they can't come into contact with the fondant at any point. If they do, the fondant will become gummy and will usually discolor from the red juice. To fill your cake with berries, first protect the layers with a thin coating of buttercream. Then, pipe a ring of buttercream around the edge of the lower layer, to act as a dam and prevent them from oozing out. Assemble the cake and cover it with another layer of buttercream, to make a smooth base for the fondant. Refrigerate it for at least an hour so the buttercream can become firm.
Remove the cake from your refrigerator, cover it with fondant, and apply your decorations. If you're making the cake 12 to 24 hours ahead, it will need to go back into the refrigerator to prevent the strawberries from softening. To minimize the risk of condensation when you take the cake out of your refrigerator, place it in front of a fan or, better yet, your air conditioner as it warms up. The cool, moving air reduces the risk of moisture spoiling your cake's finish.