Thorougly stir together flour, 3/4 cups brown sugar, baking powder andsalt. Add milk, peanut butter, oil and vanilla. Spread in a greased 8"x8" baking pan. Sprinkle with remaining brown sugar. Carefully pour hot water over entire top. Bake in 350 degree oven for 45 minutes. Serve warm with vanilla ice cream or whipped cream.