Add egg and vanilla and mix again.
Sift dry ingredients together and add to creamed mixture.
Spread into ungreased jelly roll pan. You might think there isn't enough dough, but there is...just keep spreading.
Bake at 375 for 10-12 minutes.
Meanwhile, mix frosting ingredients together until creamy and spread over bars while they are still warm. Put in the freezer for 30 minutes, cut and serve.