Grease a 9-inch square baking pan.

In a large saucepan, melt the butter over medium heat. Stir in the brown sugar and vanilla extract, then mix in the oats. Cook over low heat for 2 to 3 minutes, until the ingredients are well combined.

Press half of the oat mixture into the bottom of the prepared pan, and set the remaining mixture aside for the topping.

In a small heavy saucepan, melt the chocolate chips and peanut butter over low heat, stirring frequently until smooth. Pour the melted chocolate mixture over the oat base in the pan, spreading it evenly with a knife or spoon.

Crumble the remaining oat mixture over the chocolate layer, pressing it down gently. Cover and refrigerate for 2 to 3 hours, or overnight. Let it come to room temperature before cutting into bars.