Rhubarb is a classic Maine cooking ingredient and offers a delightfully tart experience! (Aarons S.) Mary’s Rhubarb Squares – Etta Bryant, MASP Ingredients 3 cups cut-up rhubarb 3 Tbsp. cornstarch 1 cup brown sugar nuts (if desired) 1 to 1 1/2 cups sugar 1 cup butter or margarine 1 1/2 cup oatmeal 1/2 tsp. baking