Preheat your oven to 350°F (175°C) and line an 8x8 inch baking pan with parchment paper, leaving overhang for easy removal.

Combine the graham cracker crumbs, melted butter, and granulated sugar in a bowl and stir until the mixture resembles wet sand.

Press the mixture firmly into the bottom of the prepared pan using the back of a spoon or a flat glass to create an even layer.

Bake for 8 to 10 minutes until lightly set, then remove from the oven and allow to cool slightly.

Mix the peanut butter, powdered sugar, and vanilla extract in a bowl until smooth and thick.

Spread the peanut butter mixture evenly over the slightly cooled crust.

Combine chocolate chips and 1 tablespoon of butter in a microwave-safe bowl and heat in 20-second intervals, stirring between each until fully melted and smooth.

Pour melted chocolate over the peanut butter layer and spread evenly.

Sprinkle mini marshmallows evenly across the chocolate layer.

Return the pan to the oven and bake for 5 to 7 minutes until marshmallows are puffed and lightly golden.

For a deeper golden color, optionally place under the broiler for 1 to 2 minutes, watching closely.

Remove from the oven and let bars cool completely at room temperature, then chill if needed to fully set layers.

Lift bars from the pan using the parchment overhang and slice into 12 even squares before serving.