Preheat oven to 350-degrees F. Grease a 12 x 18-inch baking sheet. Set aside. Cream the butter and peanut butter. Add white and brown sugars. Mix until creamy. Blend in egg, 2/3 cup peanut butter, baking soda, salt, and vanilla. Stir in flour and oatmeal. Spread evenly into greased baking sheet. Bake for 18-20 minutes. Do not over bake.
In a microwave-safe glass bowl melt chocolate for about 30 seconds. Stir until all chocolate is melted. Drizzle over set iced bars. NOTE: if using chocolate chips for drizzle add 1 teaspoon coconut oil or shortening with the chocolate to help thin it out.
Gluten Free Option: Substitute 2 cups all-purpose flour for 2 1/2 cups oat flour. Delish!!!!