Preparation: Preheat oven to 175°F and adjust the oven rack to the lower-middle position. Grease a 9x13-inch aluminum baking pan and warm the Rice Krispies cereal in the oven.
Bloom Gelatin: In a small bowl, mix gelatin with ¼ cup cool water and stir. Add vanilla extract and set aside.
Make Sugar Syrup: In a medium saucepan, combine ½ cup water, corn syrup, sugar, and salt. Heat on medium, stirring until bubbling (about 8 minutes), then increase heat and cook without stirring until the temperature reaches 250°F (6 minutes).
Mix Marshmallow Crème: Pour the hot syrup into the bowl of a stand mixer. Mix on low until combined, then whip on medium-high until thick and fluffy (5 minutes).
Combine and Press: Reduce the mixer speed, add melted butter, and mix briefly. Pour the marshmallow crème over the warm cereal, stir, and press the mixture into the prepared pan. Flatten evenly without crushing the cereal.
Rest: Cover the pan with foil and let rest at room temperature until firm, about 1 hour.