In a large bowl, combine the rhubarb, strawberries, 2/3 cup sugar and cornstarch. Pour into a 11-in. x 7-in. baking dish.
In a large bowl, cream the butter and remaining sugar until light and fluffy. Add egg, beat well. Beat in orange peel and vanilla. Combine the flour, baking soda and salt; add to the creamed mixture alternately with buttermilk, beating well after each addition. Spoon over fruit. Bake at 350° for 50-55 minutes or until a toothpick inserted near the center comes out clean. Cool completely on a wire rack. If desired, serve with whipped cream and additional orange zest.
