Preparation Steps

• Line a 9x13-inch baking pan with parchment paper leaving a 1–2 inch overhang on two sides; lightly coat the parchment with nonstick spray. This makes it easy to lift the finished slab out of the pan for clean slicing.
• Combine the granulated sugar and light corn syrup in a medium heavy-bottomed saucepan. Set over medium heat and stir until the sugar dissolves and the mixture is hot and just starting to bubble (about 1–2 minutes); do not let it boil vigorously.
• Remove the pan from the heat and immediately whisk in the peanut butter until the mixture is glossy and fully combined. Stir in the vanilla extract. Working quickly, fold in the puffed rice cereal until every grain is coated.
• Transfer the coated cereal to the prepared pan and press it evenly into an even layer. Use the bottom of a measuring cup or a piece of parchment to press firmly—this helps the bars hold together. Set the pan aside while you prepare the topping.
• Build a double boiler: bring an inch or two of water to a gentle simmer in a saucepan. Place a heatproof bowl that fits snugly over the pan (without touching the water) and add the butterscotch chips. Stir frequently until smooth, then transfer the melted butterscotch to a small bowl.
• Return the bowl to the double boiler and melt the semisweet chocolate chips, stirring until silky. If the chocolate thickens too much, remove from heat and stir until it loosens. Keep both melts warm but not hot.
• Pour the melted butterscotch over the pressed cereal and smooth with an offset spatula. Drizzle the melted chocolate over the butterscotch in lines and use a knife or skewer to swirl the two for a marbled finish.
• Refrigerate the pan until the topping is fully firm, about 30–45 minutes. Lift the slab from the pan using the parchment overhang and slice into 24 bars with a sharp knife. Store in an airtight container at room temperature for up to 3 days or refrigerate for longer keeping.