For super thick Rice Krispie Treats, grease a 9×9-inch baking pan with nonstick cooking spray. For regular Rice Krispie Treats, grease a 9×13-inch baking pan with nonstick cooking spray. You can also line the pan with parchment paper to make it easier for them to come out of the pan. Set pan aside.: When your pan is prepped you can focus on cooking without scrambling, and the choice of pan size directly affects bar thickness. You will notice a faint sheen of oil when the pan is properly greased, which helps prevent sticking and gives the finished edges a clean release. Using parchment creates a crisp rim and makes lifting the slab easier. One common mistake is under greasing, which causes the treats to cling and break when removed. If you hear a slight sizzle when the pan touches the spray, that grease layer is adequate.
First, brown the butter. Place the butter in a large pot and melt over medium heat, stirring often, until it foams, smells nutty, and browns. This will take about 5 minutes.: Browning the butter builds a rich, toasty aroma you will recognize immediately as the liquid turns from pale yellow to an amber color and little brown specks form. Listen for a gentle bubbling and watch for the foam to subside, then for the fat to sparkle with those warm brown bits. This step deepens the flavor far beyond plain melted butter, giving the final bars a caramel like undertone. Stirring often prevents hot spots that can burn the milk solids. A classic misstep is letting the butter go from browned to burnt, which produces a bitter scent, so pull the pot from heat as soon as you see golden flecks.
Add the 16 oz bag of marshmallows and stir until the marshmallows are completely melted.: Once the marshmallows hit the warm browned butter, they will begin to puff and collapse into a glossy, viscous mass. Keep the pot over gentle heat and stir with a silicone spatula, scraping the bottom so nothing sticks. The melted mixture should be smooth and shiny, with steam rising and a sweet aroma. If you overheat, marshmallows can become grainy or scorch, so remove from heat as soon as they are fully liquefied. A usual error is leaving the heat too high, which may cause burning at the pot base, producing off flavors.
Remove the pan from the heat and stir in the vanilla extract and sea salt. Add the Rice Krispies and Cinnamon Toast Crunch; gently stir until well coated with the marshmallows. Stir in the remaining 2 cups of mini marshmallows.: Off the heat is the best time to add vanilla extract so its aroma stays intact, and the salt will bloom against the sweet marshmallow, balancing the profile. Fold in the crispy rice cereal and Cinnamon Toast Crunch cereal with a gentle motion so the flakes remain whole and evenly coated. You will feel the mixture become straighter to the spoon and see cereal pieces glisten under a sticky sheen. The remaining mini marshmallows folded in at the end create soft pockets that contrast the crunch. Overmixing can crush the cereal and yield a dense bar, so fold just until combined.
Transfer mixture to the prepared pan. Lightly press the mixture into the pan, using a greased spatula. You can also lightly grease your hands and press the mixture into the pan. Don’t press too firmly or the treats will end up hard and dry. Sprinkle a little flaky sea salt on top, if desired.: When you move the mixture into the pan, use a light, even pressure to form a cohesive slab while preserving airiness. You want to see the surface smooth but not compacted; a glossy finish is a good visual cue. Greased hands work best for final shaping, pressing from the center to the edges without forcing out all the trapped air. The flaky sea salt on top will glint visually and provide flavorful contrast on the first bite. A frequent mistake is pressing too hard, which squeezes out aeration and yields a rock solid bar, so be gentle.
It's best to let the Rice Krispie Treats sit for about 1 hour before cutting. Cut into squares and serve.: Allowing the bars to rest lets the marshmallow set and the flavors meld, resulting in clean slices and a tender chew. You will notice the surface lose its tackiness and the edges firm slightly, which are good signs that the slab is ready. Use a sharp knife or an offset spatula to slice into even squares, wiping the blade between cuts to keep edges tidy. Cutting too soon creates sticky, misshapen pieces and makes liftoff messy, so patience here pays off.