Greas a 9"x13" pan. Cream together butter, brown sugar and sugar together. Add egg and vanilla and mix well. Separately combine flour, baking powder and baking soda before mixing in to margarine mixture.
Add cereal and raisins and stir until combined. Spread evenly in pan and bake 15-20 minutes at 350F or until it passes the toothpick test. Cool.
Combine powdered sugar and milk to form frosting and drizzle ovrer pan before cutting into squares.