Spray Pam on 9 X 13 pan. Cover bottom with marshmallows. Make cake according to package and pour over marshmallows. Spoon pie filling over batter. Bake 1 hour at 325. Let cool. Spread cool whip on the top. You can also slice fresh strawberries up to make it look pretty.
Hint: This cake tastes better when it's cold! I like to refridgerate it overnight!