Combine rhubarb and 2/3 cup sugar in a bowl. Spoon rhubarb mixture into an 8-inch square baking dish coated with cooking spray. Beat 2/3 cup sugar and butter at medium speed of a mixer until well-blended (about 5 minutes). Add egg, beating well. Beat in orange rind and vanilla. Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking powder, baking soda, and salt; stir well with a whisk. Add flour mixture to sugar mixture alternately with buttermilk, beginning and ending with flour mixture. Spoon batter over rhubarb mixture. Bake at 350° for 55 minutes or until a wooden pick inserted in center comes out clean.