Preheat oven to 350 F. Sift flour with baking powder and set aside. Cream butter and sugar until light, then beat in eggs, 1 at a time; mix in vanilla.Add flour, a few spoonfuls at a time, alternately with sour cream, beginning and ending with flour. Stir in dates and nuts. Spoon about half the batter (divided) into each of 2 greased and floured 9x5x3 inch loaf pans and sprinkle with half the brown sugar mixture. Add the remaining batter and sprinkle with the remaining sugar mixture.Bake about 1 hour or until loaf pulls slightly away from the side of the pan and is springy to the touch.Cool upright in the pan 10 minutes, then turn out on a wire rack and cool completely before slicing.Slice and serve with butter.