Cream together butter and sugar.
Add egg and mix until well blended.
Stir in banana, starter and buttermilk.
Sift flour with salt, baking powder and soda.
Add flour mixture and walnuts to the first mixture, stirring just until blended.
Pour into two greased 9 by 5 inch loaf pans.
Bake in a 350º oven for one hour or more, until a pick inserted in the center comes out clean.
Cool thoroughly on a wire rack before slicing.