Cream the butter and the two sugars together until light and fluffy. Beat in the egg, then mix in the bananas. Using a spoon now rather than beaters, mix in the sifted flour, salt and soda alternately with the buttermilk. Finally, gently stir in the chopped walnuts.

Tip the mixture into a buttered and lined 8 inch round tin, and bake at 175C for about 40 minutes, or until a skewer comes out clean.

My oven is being a cow at the moment, so I'm not sure about the exact length of time for which this should be cooked, sorry. Start checking after about 30 minutes. Here's some info on baking soda and why you shouldn't overbeat batters when you're using it.