Lightly grease a 9x13-inch baking dish or line it with parchment paper. Set aside.
In a large saucepan over medium heat, melt butter and 1 cup peanut butter, stirring until smooth and combined.
Add mini marshmallows to the saucepan and stir until fully melted and mixture is glossy.
Remove from heat and immediately fold in Rice Krispies cereal until fully coated.
Press the mixture evenly into the prepared baking dish using a spatula or wax paper without compacting too firmly.
In one microwave-safe bowl, heat remaining 1 cup peanut butter in 30-second intervals until smooth and pourable.
In another microwave-safe bowl, melt chocolate chips similarly until silky and smooth.
Pour melted peanut butter evenly over the cereal base.
Drizzle melted chocolate over the peanut butter layer and use a toothpick or butter knife to swirl both into a marbled pattern.
Allow bars to set at room temperature for 1 to 2 hours or refrigerate for about 45 minutes until the topping is firm.
Once set, cut into squares with a sharp knife, wiping the blade between cuts for clean slices.
Serve immediately or store in an airtight container.