Preheat oven to 350 degrees F (175 degrees C). Grease and flour two 9-inch round cake pans or one 9x13-inch pan.
In a medium bowl, cream together white sugar and 1/2 cup butter. Mix in eggs and 3 mashed bananas.
Sift together flour and baking soda in a separate bowl. Add to the creamed mixture alternately with buttermilk, mixing well after each addition. Blend in 1 teaspoon vanilla extract. Fold in 1 cup of pecans and 1 cup coconut. Pour batter into prepared pans.
Bake in the preheated oven until a tester inserted in the center comes out clean, about 45 minutes. Cool completely on wire racks.
To Make Frosting: Cream together 1/2 cup butter and confectioners' sugar until light and fluffy. Mix in 1 medium mashed banana, 1 cup pecans, 1 cup coconut and 1 teaspoon vanilla.