Preheat oven to 375 degrees F.
Butter a 9 by 13 inch pan. In a large bowl, mix the oatmeal, brown sugar, butter, vanilla and corn syrup. Spread the mixture into the prepared pan and bake 15 minutes. Cool for 10 minutes. Cut into bars.
For the Frosting: Combine chocolate chips and peanut butter in a double boiler and melt. Mix well. Spread on baked and cooled cookie bars. Refrigerate until frosting hardens.