Combine brown sugar, flour and butter in mixing bowl; mix by hand. Remove and reserve one cup brown sugar mixture for frosting. Cream remaining mixture in bowl until light and fluffy. Add egg, buttermilk, and baking soda; beat well. Stir in raisins and walnuts. Beat in vanilla. Pour into butter 9 x 9" pan. Spread with reserved mixture. Bake at 350 degrees for 40 minutes or until cake tests done.