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.
This is excellent, have made it multiple times, great for company. Usually serve with frozen french vanilla yogurt instead of the whipped topping. It is on our favourites list.