Grease an 8×8 pan with butter, then coat it generously with powdered sugar.

Mix the gelatin with ½ cup of cold water and set it aside for about 10 minutes to bloom.

Add the remaining water and sugar to a saucepan over medium heat. Stir until the sugar dissolves, then continue heating until it reaches 230°F.

Pour the hot sugar mixture into a mixer fitted with the whisk attachment, then add the gelatin mixture.

Mix on low speed for 2 minutes, then increase to high and whip for 5 minutes. Add the vanilla and salt after the 5 minutes, then mix again on high for 5 more minutes until thick, fluffy, and glossy

Pour the marshmallow mixture into the prepared pan, smooth the top, and dust with more powdered sugar.

Let it set uncovered for at least 2 hours, then slice into marshmallows.