Best Ever! Peanut Butter Bars

I love any excuse to make these... (I need an excuse b/c they make such a large pan we can't eat them all. Well, we shouldn't. Well, I shouldn't...)

3/4 cup margarine
3/4 cup peanut butter
3/4 cup brown sugar
3/4 cup white sugar
2 eggs
3/4 tsp soda
1/2 tsp salt
1 1/2 tsp vanilla
1 1/2 cup flour
1 1/2 cup oatmeal

Cream wet ingredients, then mix in dry ingredients. Spread into an 11 x 17 rimmed baking sheet. Bake at 375 or 12-15 minutes. While still hot, spread with about 1 cup peanut butter (I don't measure this). Frost when cool.

Frosting:
1/4 cup margarine
1/3 cup cocoa
1/4 tsp salt
1/3 cup milk
1 1/2 tsp vanilla
Powdered sugar to desired consistancy