What Flavour Birthday Cake Should I Have What Could I Put In The Middle Of A Birthday Cake??? Other Than Frosting?

What could I put in the middle of a birthday cake??? Other than frosting? - what flavour birthday cake should i have

I bake a cake for the first time today and I like the taste ... what could go in the middle? Jam? Cream cheese? The race is the vanilla flavor and I try to do a Sponge Bob cake ... not so important to this detail.