Mix gelatin and ½ cup of the water in the bowl of an electric mixer. Prepare the sugar mixture and while you are doing this the gelatin will bloom.
In a small saucepan, bring ½ cup of water, sugar, and light corn syrup to a boil.
Boil until it reaches 240°F and then remove the pan from heat (about 6 minutes).
With the mixer on medium speed and using the whisk attachment, slowly pour the hot liquid into the gelatin.
Increase the speed to high and beat mixture until it becomes white and fluffy (about 5 minutes)
Turn the mixer to low and add in salt, vanilla, and melted butter. Allow to mix on high for another minute or so and then remove the bowl from the stand mixer.
Add in Rice Krispies, crumbled Oreo Cookies (reserve about 1 cup for topping), and mini marshmallows and stir by hand.
Stir until everything is well-coated, this should take about a minute and a little elbow grease.
Transfer the mixture to the prepared baking dish. Press mixture down a little; then, sprinkle remaining Oreos on top. Use your hand to press the bars down so that they are all the same height.
Refrigerate for 10-15 minutes (or up to overnight) to make sure the marshmallows set.
Serve and enjoy!