Line a 9x13-inch baking dish with parchment paper and set aside.
Pour ½ cup of the cold water into the bowl of a stand mixer. Sprinkle the gelatin evenly over the water and let it sit while you prepare the honey syrup.
In a small saucepan, combine the remaining ½ cup of water, honey, and sea salt.
Bring the mixture to a gentle boil over medium heat. Simmer for 7-8 minutes, until slightly thickened.
Turn the mixer on low speed. Carefully and slowrly pour the hot honey syrup into the gelatin mixture.
Gradually increase the mixer speed to high. Whip for 8-10 minutes, until thick, glossy, fluffy, and marshmallow-like. Add the vanilla extract during the last minute of mixing.
While the marshmallow is whipping, melt the butter in a large pot over low heat.
Add the whipped marshmallow mixture to the melted butter and stir until combined.
Fold in the crispy rice cereal until evenly coated.
Transfer the mixture to the prepared baking dish. Using lightly buttered hands or a piece of parchment paper, gently press into an even layer.
Sprinkle with flaky sea salt if desired. Let cool for about 30 minutes before slicing into squares and serving.
Notes
The optional flaky sea salt turns this into an amazing salty sweet treat.
Try drizzling the top with some melted dark chocolate for a more decadent dessert.