• Line a 9x13-inch baking dish with parchment paper, ensuring the paper goes up the sides of the dish for easy removal later.
• To a medium bowl, add the crushed Oreo cookies, sugar, and melted butter. Mix until fully incorporated with no dry spots remaining.
• Press the mixture firmly and evenly into the bottom of the lined baking dish. Set aside.

Caramel

• In a large saucepan over medium heat, melt the butter.
• Add sweetened condensed milk, heavy cream, corn syrup, brown sugar, salt, and vanilla. Stir until combined. Continue cooking, stirring constantly, until the mixture reaches 236°F on a candy thermometer (about 15-18 minutes).
• Carefully pour the caramel (the mixture will be very hot) over the Oreo crust, spreading it evenly. Be careful not to lift the crust. Set aside.

Marshmallow

• In the bowl of a stand mixer with a whisk attachment, add the gelatin. Pour ½ cup of the cold water over the gelatin and let it sit while you prepare the sugar syrup.
• In a small saucepan over medium-high heat, pour in the remaining cold water, sugar, corn syrup, and salt. Cover and cook for 3-4 minutes, or until the sugar begins to dissolve.
• After 4 minutes, uncover the saucepan and fit with a candy thermometer. Continue to cook until the thermometer reads 240°F (5-7 minutes). Remove from heat. (If you don’t have a candy thermometer, bring the mixture to a rolling boil and let it boil for one minute before removing from heat.)
• With the mixer on low speed, slowly pour the sugar mixture into the gelatin mixture. After all the sugar mixture has been added, increase the speed to high. Continue to whisk on high for 10-15 minutes, or until the mixture is very thick. At the last minute, add the vanilla. (For a stiffer marshmallow, whisk the full 15 minutes.)
• While the marshmallows are being whisked, add the chocolate chips to a medium microwave-safe bowl. Melt in the microwave in 20-second increments, stirring in between, until smooth and melted.
• When ready, pour the marshmallow mixture on top of the caramel layer. Using a spatula that has been sprayed with cooking spray, spread the mixture out to the corners.
• Pour or dollop the chocolate over the marshmallow mixture. Use a knife to swirl the chocolate into the marshmallow layer, creating large visible streaks.
• Allow the bars to set at room temperature for at least 4 hours, up to overnight, until fully set.
• Serve at room temperature for best results.