Preheat the oven to 350°F (175°C) and grease a 9×9-inch baking pan.
Combine melted butter and brown sugar in a mixing bowl.
Add the egg and vanilla extract, stir until smooth.
Gradually mix in the flour to form a thick batter.
Spread the batter evenly in the prepared baking pan.
Melt the chocolate chips and peanut butter together, then pour over the batter.
Sprinkle chopped peanuts on top if desired.
Bake for 25-30 minutes until edges are lightly golden.
Allow to cool completely before cutting into squares and serving.
