In a large bowl, combine 3/4 cup sugar, cornstarch and 2 teaspoons cinnamon, mix well. Add rhubarb and strawberries, toss well and transfer the mixture to a buttered 11x7-inch baking dish or 1-1/2 quart shallow casserole dish. In a small bowl, combine flour, baking soda and salt, set aside. With an electric mixer, beat together butter and 1/3 cup sugar until light and fluffy. Beat in egg. On low speed, beat in half of flour mixture. Beat in buttermilk, then remaining flour mixture. Drop batter by heaping tablespoonfuls into 8 mounds over fruit. If desired, sprinkle cinnamon-sugar topping over batter. Bake in a 375°F oven 35 to 40 minutes or until fruit is bubbly and topping is golden brown. Transfer to a wire cooling rack. Let stand at least 40 minutes before serving. Serve warm or at room temperature with ice cream or whipped cream, if desired. Yield: 8 servings. (Recipe is easily doubled in two baking dishes to serve 16).