Line an 8-inch square baking dish with parchment paper.

In the bowl of an electric mixer, combine gelatin and 0.5 cup of water. Let it bloom while preparing the sugar mixture.

In a small saucepan, bring 0.5 cup of water, granulated sugar, and light corn syrup to a boil.

Boil until the mixture reaches 240°F (115°C). Remove from heat.

With the mixer on medium speed and using the whisk attachment, slowly pour the hot liquid into the gelatin mixture.

Increase the mixer speed to high and beat until the mixture is white and fluffy, about 5 minutes.

Turn the mixer to low speed and add kosher salt, vanilla extract, and melted butter. Mix on high speed for another minute.

Gently fold in the Rice Krispy cereal, crumbled Oreo cookies (reserving about 1 cup for topping), and mini marshmallows by hand until everything is well coated.

Transfer the mixture to the prepared baking dish. Press down gently and evenly. Sprinkle the reserved Oreo crumbs on top and press them in.

Refrigerate for 10-15 minutes, or until firm.

Cut into bars and serve.