Step 1: Prepare pan and melt butter: Grease a 9 x 9 inch square pan. Melt 1 cup butter in a large saucepan over medium heat.
Step 2: Make the Oatmeal Mixture: Stir 1 cup brown sugar and 1 teaspoon vanilla extract into the melted butter. Stir in 1 cup instant oatmeal. Cook over low heat for 2 to 3 minutes, or until ingredients are well blended.
Step 3: Form the Crust and Save the Topping: Press half of the oatmeal mixture into the bottom of the prepared pan to form the crust. Save the other half for the topping.
Step 4: Make the chocolate filling: Meanwhile, melt 225g semi-sweet chocolate chips and 115g peanut butter in a small, heavy saucepan over low heat, stirring constantly until smooth.
Step 5: Layer and chill: Pour the chocolate mixture over the crust in the pan and spread evenly with a knife or the back of a spoon. 2 Crumble the remaining oatmeal mixture over the chocolate layer and press gently. 3 Cover the pan and chill for 2 to 3 hours or overnight until firm.
Step 6: Slice and serve: Let the cookies come to room temperature before cutting into squares.
