Pour butter into an 8-in.-square baking dish. Sprinkle with brown sugar; top with drained rhubarb. Sift flour, baking powder and salt together twice.
In a large bowl, beat eggs on high speed 3 minutes. Gradually add sugar, beating until thick and lemon-colored. Beat in extract and reserved cooking liquid. Fold in flour mixture. Pour over rhubarb. Bake 35-40 minutes or until top springs back when lightly touched.
Cool 10 minutes before inverting onto a serving plate. Serve warm.