Stir together 2 cups flour and brown sugar. Cut in butter until mixture resembles coarse meal. Measure 3/4 cup of mixture and set aside for topping. To remaining flour mixture add 1/3 cup flour; blend well. Add egg, buttermilk, baking soda, and cinnamon. Mix just until dry ingredients are moistened.
Turn dough into a buttered 9-inch square pan. Mix together the reserved flour sugar mixture and the chopped walnuts or pecans; sprinkle over the top of the batter. Bake coffee cake in a 350° oven for 50 to 55 minutes.
Let coffee cake cool in pan then cut into serving-size pieces.