Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper or foil, leaving an overhang for easy removal. Lightly grease the lining.

In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth. Stir in the beaten egg and vanilla extract until fully combined. Add the flour, baking soda, and salt. Mix until just combined—do not overmix. Press about ⅔ of the dough evenly into the bottom of the prepared pan. Reserve the remaining dough for the topping.

In a medium bowl, combine the peanut butter, softened butter, and powdered sugar. Mix until smooth and creamy. Spread the peanut butter mixture evenly over the cookie base, leaving a small border around the edges.

Crumble the reserved dough over the peanut butter layer to create a crumbly topping.

Bake for 25–30 minutes, or until the edges are golden brown and the center is set. Let the bars cool completely in the pan on a wire rack before adding the chocolate topping.

In a microwave-safe bowl, combine the chocolate chips and butter. Microwave in 30-second intervals, stirring between each, until smooth and melted. Spread the melted chocolate evenly over the cooled bars. Let the chocolate set at room temperature or in the fridge for 15–20 minutes.

Once the chocolate has set, lift the bars out of the pan using the parchment paper or foil overhang. Cut into squares or rectangles.