PREHEAT OVEN TO 180ºC . Sift together the flour, baking powder, baking soda, salt and cocoa. Cream the butter, add the vanilla and sugar, beat until light, about 5 minutes. Add the egg. Add the dry ingredients in three additions, alternating with the buttermilk. Do not over mix. Stir in the currants and nuts. Pour into a greased and floured 7-by-5-by-3-inch loaf pan. Bake for 1 hour 10 minutes. Cool in the pan 10 minutes, remove and cool on a rack.