First, prep the pan. Grease a small square 8x8 baking dish with butter and dust the bottom with powdered sugar.
Next, bloom the gelatin. In the bowl of a stand mixer, add ½ cup cold water and sprinkle the gelatin on top. Let it sit and bloom for 5 minutes.
Now, make the sugar syrup. In a small saucepan, combine 1 cup cold water, sugar, and a pinch of salt. Heat over medium, stirring until dissolved, then bring to a gentle boil. Let it boil for 1 minute, then remove from heat. Mixture must be at 240F.
Next, mix it. Carefully pour the hot sugar mixture into the bloomed gelatin. Using the whisk attachment, beat on high for 10-15 minutes until the mixture becomes thick, glossy, and tripled in volume.
Add vanilla extract and mix for another 10 seconds. Immediately transfer the marshmallow mixture to the prepared pan and smooth the top. Dust with more powdered sugar.
Let it set at room temperature for 3–4 hours.
Finally, cut it. Once firm, turn out onto a powdered-sugar–coated surface, dust the top again, and cut into cubes. Toss the cubes lightly in powdered sugar to prevent sticking.