Preheat the oven to 180C/350F/gas mark 4
Use a 23cm/9 inch square cake tin.
Grease and line the cake tin. Beat together the butter and the sugar until creamy. Add the sour cream and beat well. Sift the dry ingredients together, then sift again over the raisins and toss to mix through. Add alternate batches of the flour-raisin mixture and the buttermilk to the butter cream, about a third of each at a time. Fold in lightly but thoroughly. Do not overmix. Spoon the mixture into the cake tin, spreading evenly.Bake for an hour or until a warm knife inserted into the centre comes out clean.
