4tablespoons½ stick unsalted butter, plus more for greasing the pan
½teaspooncoarse Kosher salt
7cupscrispy rice cereal
For the caramel topping:
¼cuplight corn syrup
Sea salt flakes
Making the crispy treats:
Grease an 8 x 8 or 9 x 9 square pan with butter. Place a sheet of parchment paper in the bottom of the pan, leaving a 1-2 inch overhang on two sides. Grease the bottom and sides of the parchment paper with butter as well.
Place the 4 tablespoons of butter in a large, heavy bottomed saucepan. Melt over medium heat. Stirring frequently, melt and cook the butter until brown bits form at the bottom of the pan and the butter smells fragrant and nutty. It will foam and then turn clear before it starts to brown. Once it is brown and fragrant, remove from heat immediately to prevent burning. Stir the marshmallows into the hot butter until they are melted and smooth (you may need to put the pot back on very low heat to get them completely melted). Stir in the salt and vanilla. Stir in the crispy rice, tossing until all the pieces are coated.
Spread the crispy rice mixture into your prepared pan. Press down on the mixture to firmly press it evenly into the pan (it helps to coat your fingers in a little butter before doing this so the mixture doesn’t stick to your hands).
Set aside and allow to cool to room temperature for about 30 minutes. While the crispy rice treats set, make the caramel.
Making the caramel:
Place the sugar, corn syrup, and water in a medium size, heavy bottomed saucepan (try to avoid using a dark coated pot so you can more accurately assess the color of the caramel). Fit the sides of your pan with a candy thermometer. Bring to a boil over medium high heat, stirring just until the sugar dissolves. Once the sugar dissolves, stop stirring and continue to boil the mixture. Dip a pastry brush in water and use this to wash down any residual sugar that may be on the sides of your pan into the boiling mixture - this prevents the sugar from crystallizing in the mixture as it cooks, which could result in a grainy caramel.
Keep a close eye on the sugar mixture as it boils. You will begin to see streaks of amber color forming on the bottom of the pan. This indicates that the water is evaporating off and the sugar is starting to caramelize. Resist the temptation to stir! If you see streaks of your mixture become dark in color, gently use the pot handle to swirl the bottom of the pan to distribute the dark color, being careful not to slosh the mixture up the sides of the pot (you can use the wet pastry brush to wipe down the sides of pot if this happens).
When the mixture is light amber in color and reaches 320 degrees F, remove the pot from heat. Slowly whisk in the ½ cup of heavy cream, being careful to avoid splatters as the caramel will be EXTREMELY hot (the caramel will bubble and sputter as you add the cream). Stir in the butter and vanilla. Allow the caramel to cool for 10 minutes before pouring.
Use the parchment paper overhang to remove the crispy rice treats from the pan once they have set. Place the parchment on a cutting board. Pour the warm caramel on top of the crispy rice square. Sprinkle with flakes of sea salt. Allow the caramel to set for another 10 minutes before slicing - if you can wait that long! Serve warm or store cut squares in an airtight container. They will stay fresh for 3-5 days.