Preheat oven to 350. Grease and flour 9 x 13 inch casserole dish or cake pan.
In bowl combine eggs and sugar. Mix well. Add butter, 3 bananas and buttermilk. Mix well. Add baking powder, soda, 1 tbsp. vanilla, flour and 1/2 cup chopped pecans. Mix well and spread batter in bottom of dish. Bake 35-40 minutes or until toothpick poked in center comes out clean.
In another bowl make frosting. Add box of powdered sugar, 1/2 stick butter, 1 banana and 1/2 tsp vaniila. Mix well. Spread on top of cooled caked. Sprinkle with the other 1/2 cup of chopped pecans