01 - Line an 8x8-inch baking pan with parchment paper, leaving enough overhang on the sides to lift the bars out easily after setting.
02 - In a medium saucepan over low heat, combine the creamy peanut butter and unsalted butter. Stir continuously until completely smooth and fully melted.
03 - Remove the saucepan from heat. Add the gluten-free powdered sugar and pure vanilla extract, stirring vigorously until the mixture is well combined and no lumps remain.
04 - Allow the mixture to cool for 2 to 3 minutes so it is still warm but not piping hot. Gently fold in the mini marshmallows, coating them evenly without letting them melt completely.
05 - Pour the marshmallow-peanut butter mixture into the prepared baking pan. Use a spatula or your hands to press it into an even, uniform layer.
06 - If using, sprinkle the gluten-free chocolate chips across the surface and lightly press them into the mixture so they adhere.
07 - Refrigerate the pan for at least 1 hour, or until the bars are firm and fully set throughout.
08 - Lift the slab out of the pan using the parchment overhang. Cut into 16 even squares and serve chilled.