Line a 9x9-inch baking pan with parchment paper.

In a medium saucepan over medium heat, melt the butter. Stir in the brown sugar and vanilla. Next, add in the oats and cook over low heat 2 to 3 minutes.

Press half of mixture into the bottom of the prepared pan. Reserve the other half for topping.

In a small saucepan over low heat, melt the chocolate chips and peanut butter, stirring frequently until the mixture is smooth.

Pour the chocolate and peanut butter mixture onto the crust layer and spread evenly with an offset spatula or spoon.

Crumble the remaining oat mixture over the chocolate layer, gently pressing it in. If desired, sprinkle with mini chocolate chips.

Cover, and refrigerate for at least 4 hours. Allow bars to sit at room temperature for about 30 minutes before cutting into bars.