Line a 9x13-inch baking dish with parchment paper, leaving some overhang for easy removal later.

In a medium bowl, combine the crushed Oreo cookies, sugar, and melted butter until well mixed. Press this mixture firmly into the bottom of the lined baking dish and set aside.

In a large saucepan over medium heat, melt the butter. Add in the sweetened condensed milk, heavy cream, corn syrup, brown sugar, salt, and vanilla. Stir until the mixture reaches 236°F on a candy thermometer, about 15 to 18 minutes.

Carefully pour the caramel over the prepared Oreo crust, spreading it evenly.

In the bowl of a stand mixer, sprinkle the gelatin over 1/2 cup of cold water and let it sit.

In a small saucepan, combine the remaining 1/2 cup of cold water, granulated sugar, corn syrup, and salt. Cook over medium heat, stirring until sugar dissolves, then cover and let it boil until it reaches 240°F, about 5 to 7 minutes.

Slowly pour the hot sugar mixture into the gelatin while the mixer is on low speed. Once well combined, increase the speed to high and whisk for 10 to 15 minutes until thick, adding in the vanilla during the last minute of whisking.

Pour the marshmallow mixture over the caramel layer and smooth it out evenly with a spatula.

Melt the chocolate chips in the microwave in 30-second intervals, stirring until smooth. Drizzle the melted chocolate over the marshmallow layer, swirling it together as desired.

Allow the bars to set at room temperature for at least 4 hours or ideally overnight for the best texture.

Lift them out using the parchment paper and cut into bars. Serve these at room temperature.