Combine the dry ingredients and mix thouroughly to distribute the soda and baking powder, then add enough buttermilk to make a soft dough but firm enough to hold its shape. Knead on a lightly floured board for 2 or 3 minutes, until quite smooth and velvety. Form into a round loaf and place in a well buttered 8 inch cake pan or on a well buttered cookie sheet. Cut a cross on the top of the loaf with a very sharp floured knife. Bake in a preheated 375F oven for 35-40 minutes, or until the loaf is nicely browned and sounds hollow when rapped with the knuckles.
(The cross will have spread open which is characteristic of soda bread.)
From _Beard on Bread_.