Line a 8x8 pan with parchment paper, leaving an overhang on the sides, and set it aside.
Add the crispy rice cereal and an optional extra handful of mini marshmallows into a large mixing bowl.
Melt the butter in a large saucepan over medium-low heat until completely liquid. Add the 10 ounces of mini marshmallows to the saucepan and stir continuously over low heat until fully melted, smooth, and fluffy.
Remove the saucepan from the heat, add the peanut butter powder and vanilla bean paste, and stir thoroughly until completely smooth and combined. Pour the warm peanut butter marshmallow mixture directly over the crispy rice cereal in your bowl.
Working quickly before the marshmallow sets, use a greased spatula to fold everything together until all cereal pieces are evenly coated.
Transfer the mixture into your prepared 9x9 pan and press down lightly and evenly with a spatula without packing the bars too firmly.
In a small microwave-safe bowl, combine the chocolate chips and 3 tablespoons of peanut butter. Microwave the mixture in 30-second increments, stirring well between each burst, until completely melted and glossy.
Pour the melted chocolate peanut butter topping evenly over the cereal layer in the pan and gently spread it to the corners. Immediately scatter the chopped Reese’s cups across the warm chocolate layer.
Sprinkle the flaky sea salt evenly across the top to finish.
Allow the treats to cool and set completely at room temperature for at least 1 to 2 hours before using the parchment overhang to lift them out, slicing into 16 squares, and serving.