You can add everything from sliced bananas to a fruit compote to a pastry filling to your cake by spreading the filling between the layers of a cake. For single layer cakes, use a long-bladed serrated knife to slice the cake horizontally.
A very thin layer of frosting spread on the lower layer and the bottom of the upper layer helps seal the cake and helps keep the cake from getting soggy.
Before spreading the filling on the bottom layer, make a dam with your frosting to help hold the filling in. The easiest way is to pipe frosting around the perimeter to a depth of 3/8 inch or more. This dam also makes it easier to frost the outside of the cake without smearing the filling into the frosting.