Preheat your oven to 350°F (175°C) to ensure even baking.
Grease your 9x13 inch baking pan with butter or non-stick spray, or line it with parchment paper.
In a mixing bowl, combine the melted butter, brown sugar, and vanilla extract. Stir until well combined, then add the egg and whisk until smooth.
In a separate bowl, whisk together the flour and salt. Gradually add to the wet mixture, stirring until just combined.
Gently fold in the chopped graham crackers and chocolate chips.
In a small saucepan over low heat, combine the peanut butter and condensed milk. Stir until smooth.
Spread half of the batter in the baking pan, pour the peanut butter mixture over it, and then add the remaining batter on top.
Bake for 25-30 minutes, until the edges are golden brown and the center is set.
Cool completely in the pan on a wire rack, then slice into bars and serve.
