Line a 9x13-inch baking dish with parchment paper, leaving some overhang on both sides. This will make it easier to lift the treats out of the pan later.
In a large saucepan over medium heat, melt the unsalted butter and maple syrup until completely melted and bubbles form.
Add the mini marshmallows to a saucepan and stir constantly until they completely melt and the mixture becomes smooth and sticky.
Stir in the creamy peanut butter until fully combined with the melted marshmallow mixture. Mix in the pure vanilla extract to enhance the flavor of the mixture.
Remove the saucepan from the heat and quickly fold in the Rice Krispie Cereal until evenly coated with the marshmallow mixture. The mixture will start to set, so work swiftly.
Transfer the cereal mixture to the prepared baking dish and work fast to shape and press it firmly and evenly into the pan. To prevent sticking, moisten your hands with spray cooking oil or use a damp spoon to smooth the mixture.
Melt the milk chocolate chips in the microwave in 30-second intervals, stirring between intervals until smooth, or use a double boiler. If using a double boiler, place the chocolate chips in a heatproof bowl set over a pot of simmering water. Stir occasionally until the chocolate has melted.
Pour the melted chocolate over the pressed cereal mixture, spreading it evenly with a spatula.
Sprinkle a pinch of sea salt over the chocolate layer.
Place the dish in the refrigerator for about 30 minutes or until the chocolate is set and the treats are firm.
Once set, remove the treats from the refrigerator, cut them into bars, and serve.