Stir together flour, baking soda, salt, cinnamon, nutmeg, and allspice.
In mixing bowl cream butter and sugar together. Add eggs, buttermilk, vanilla, and baking powder and mix together. Add dry ingredients and mix well.
Add bananas and nuts to mixture and pour into greased and floured 9 x 13 pan. Bake at 375 degrees for approximately 30 minutes. Test with toothpick to be sure cake is done.