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.