01 - Line an 8x8-inch baking pan with parchment paper, leaving enough overhang on the sides to lift the squares out easily after setting.
02 - In a medium saucepan over low heat, combine the creamy peanut butter and unsalted butter. Stir continuously until fully melted and smooth, about 2 to 3 minutes.
03 - Remove the saucepan from heat. Add the gluten-free powdered sugar and pure vanilla extract, stirring vigorously until the mixture is completely smooth and well combined.
04 - Allow the mixture to cool for 2 to 3 minutes so the heat is not intense enough to melt the marshmallows. Gently fold in the mini marshmallows, coating them evenly without stirring too aggressively.
05 - Pour the marshmallow-peanut butter mixture into the prepared baking pan. Use a spatula or the back of a spoon to press it into an even, uniform layer.
06 - If using, scatter the gluten-free chocolate chips across the surface and gently press them into the mixture so they adhere.
07 - Place the pan in the refrigerator and chill for at least 1 hour, or until the squares are firm and hold their shape when pressed.
08 - Lift the parchment overhang to remove the block from the pan. Cut into 16 even squares using a sharp knife. Serve chilled for the best texture.