Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, baking powder, and salt, stirring well with a whisk. Place butter in a large bowl, and beat with a mixer at medium speed until smooth. Add 2/3 cup granulated sugar; beat until well blended. Add vanilla, cinnamon, and egg, beating well. Beating at low speed, add flour mixture and milk alternately to sugar mixture, beginning and ending with flour mixture; beat just until smooth.
Combine rhubarb and remaining 2/3 cup granulated sugar in an 8-inch square baking dish coated with cooking spray. Spoon batter over rhubarb mixture. Bake at 350° for 45 minutes or until a wooden pick inserted in center comes out clean. Sprinkle with powdered sugar.