Prepare the Pan: Line a 13×9 inch pan with parchment paper, leaving some extra on the short sides for easy removal, or spray the pan with baking spray.
Melt the Ingredients: In a large microwave-safe bowl, combine butter, mini marshmallows, and peanut butter. Microwave until melted.
Mix the Base: Remove from the microwave, stir in vanilla extract, and then add the Rice Krispies, mixing until fully combined.
Press into the Pan: Press the mixture evenly into the prepared pan using greased hands to prevent sticking.
Melt the Chocolate: Place chocolate chips or chopped chocolate in a heatproof bowl. Microwave in 30-second intervals on medium power, stirring in between, until almost melted. Stir until smooth, then add coconut oil and mix well.
Assemble the Topping: Pour the melted chocolate over the Rice Krispies mixture. Sprinkle with chopped Reese’s cups and a pinch of sea salt for extra flavor.
Set the Bars: Let the bars set at room temperature for about 2 hours, or speed up the process by refrigerating them for 30 minutes until firm.
Serve: Once hardened, cut the bars into squares and serve!