Scotcheroos

Lightly coat a 13x9" baking pan with nonstick vegetable oil spray, then line with parchment paper, leaving generous overhang on the long sides. Place 6 cups puffed rice cereal in a large bowl.

Cook ½ cup (packed; 100 g) dark brown sugar, 1 cup light corn syrup, and 2½ tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt in a small saucepan over medium-high heat, stirring, just until sugar is melted, about 3 minutes. (Do not bring to a rapid boil.) Remove from heat, add 1⅓ cups creamy peanut butter and 1 Tbsp. vanilla extract or vanilla bean paste, and stir until peanut butter is melted and combined. Immediately pour over puffed rice cereal and, working quickly, stir to evenly coat cereal. Scrape into prepared pan and gently press into a flat, even layer, taking care not to smash the cereal too much. (Pressing it in too hard can result in dense bars.) Set aside.

Microwave 12 oz. semisweet chocolate chips, ¼ tsp. Diamond Crystal or Morton Kosher salt, and 6 Tbsp. water in a medium microwave-safe bowl on high power in 15-second bursts, stirring after each, until melted, about 1 minute. Whisk vigorously until smooth and pour over bars and spread into an even layer. (Alternatively, heat in a heatproof bowl set over a saucepan of simmering water; do not let bowl touch water. Heat, stirring, until smooth.)

Microwave 6 oz. butterscotch chips, remaining ¼ tsp. Diamond Crystal or Morton Kosher salt, and 2 Tbsp. water in another medium microwave-safe bowl on high power in 15-second bursts, stirring after each, until melted, about 1 minute. Whisk vigorously until smooth. (Alternatively, heat in a heatproof bowl set over a saucepan of simmering water; do not let bowl touch water. Heat, stirring, until smooth.) Drizzle over chocolate layer in thin streaks. Using a chopstick or thin knife, swirl butterscotch mixture into chocolate layer, being careful not to overmix. Let sit at room temperature at least 6 hours.

To serve, sprinkle flaky sea salt over scotcheroos if desired and cut into a 6x4 grid to make 24 squares. Do Ahead: Bars can be made 1 week ahead. Store tightly wrapped at room temperature, or freeze in resealable plastic freezer bags up to 3 months.