Cover cranberries with boiling water and leave to soak for 15 minutes. Strain and discard liquid. Preheat oven to 180c. Grease a loaf tin well with butter. Cream the butter and sugar until pale. Add eggs one at a time beating continuously. Sift in the flour, baking soda, and salt. Add mashed banana, cranberries, and buttermilk then stir to combine. Transfer to the baking tin. Bake for 30-40 minutes until a skewer comes out clean.