Cream butter and sugar. Add eggs, one at a time, beating after adding each. According to Cook's Illustrated test, this part is important, leads to a fluffier cake. Add vanilla, bananas & sour cream, blend til smooth. Add dry ingredients, beat well til thoroughly mixed. Pour into a greased (or use cooking spray) 9x13 pan, bake at 325 for 10 minutes, then reduce temperature to 300 for additional 45 minutes. Cool completely before icing. Best with cream cheese icing...recipe also on this site.