Fruits, jams, jellies, peanut butter, fudge, whatever you can think of usually can be put inside of a cake. Everyone likes a surprise filling inside that pretty white iced cake. How you fill your cake depends on what you are filling it with. I recommend if you are filling you cake to first add a circle of icing to the outer edge of your cake, so nothing leaks out. Some people like to add a entire layer of buttercream in the middle than a thin layer of peanut butter or jam over it. Some people just want a thick layer of fudge or mousse with no icing under it. So it is basically up to you how much of the flavor you would like in the center of your cake on what you do.