Cream together butter and brown sugar until light. Add egg and vanilla, and mix well. Stir together flour, salt, and baking powder and add to dough, mixing well. Stir in nuts. Form dough into a 2-inch-diameter log and wrap in foil or waxed paper. Chill for 12 to 24 hours. Slice and bake on ungreased cookie sheets at 400 degrees F for 8 to 10 minutes.