For topping, in a bowl stir together 1/2 cup brown sugar and 1/2 cup flour, cut in 1/4 cup peanut butter and 3 tablespoons margarine or butter till crumbly. Set aside.
In a bowl stir together 2 cups flour, 1 cup brown sugar, the baking powder, baking soda, and salt. Add milk, 1/2 cup peanut butter, eggs, and 1/4 cup butter. Beat with an electric mixer on low speed till blended. Beat at high speed for 3 minutes, scraping the sides of the bowl frequently.
Pour batter into a greased medium baking pan, spreading evenly. Sprinkle with topping mixture. Bake in oven about 30 minutes.