I'm a Pennsylvania native and got a recipe from my mom for a lemon sponge pie.  Did a Google search and apparently it's a Pennsylvania Dutch recipe and not well known outside of PA (at least that's what I read on line).  As the pie bakes, a cake-like layer will form on top of a creamy lemon filling.  If you like lemon, this is a pie for you!
I got this recipe from Recipesource (I think).  I actually like it better than my Mom's.  It has twice the lemon and half the sugar of hers.
1/2 cup sugar
2 T. flour (According to my Mom's recipe, you can use anywhere from 2 to 5 T. of flour.  The more flour, the thicker the cake topping.)
2 lemons, juiced and rinds grated
2 egg yolks, beaten
1 T. melted or very soft butter
1/4 t. salt
1 cup milk
2 egg whites
1 pie shell (8 or 9 inches) 
Mix flour and sugar.  Add lemon juice and grated rind, yolks, salt, and butter.  Add milk.  
In separate bowl beat egg whites until firm and then fold into batter.  Pour in crust and bake at 400 for 10 minutes.  Reduce temp to 350 and bake for 30 minutes or until pie springs back from light touch.