Preheat oven to 350°.
Combine brown sugar and 1 tablespoon flour in a small bowl. Lightly spoon 1 1/4 cups flour into dry measuring cups; level with a knife. Combine with granulated sugar, 1/3 cup pecans, baking powder, and salt in a medium bowl; make a well in center of mixture. Combine milk, margarine, and vanilla in a bowl; add to flour mixture. Stir just until moist.
Spread batter into an 8-inch square baking pan; sprinkle with brown sugar mixture. Pour boiling water over batter (do not stir). Bake at 350° for 35 minutes or until pudding is bubbly and cake springs back when touched lightly in center. Serve warm with whipped topping. Garnish with pecans, if desired.