Certain fillings are firm and stable enough to be spread across the layers of a cake. They won’t seep out in between the layers. Thinner fillings, like pastry creams and fruit purées, are so soft that, if not sealed, they will leak out and down the sides of the cake.
Do create a border for the filling. Use the same frosting you plan to use for the outside of the cake, and pipe a single ring around the outer edge of the cake layer. Then spread the filling inside the border; repeat this step for each layer of filling.
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