Toast the marshmallows until deeply golden brown.
Add the toasted marshmallows, water, granulated sugar and brown sugar to a saucepan and heat over medium-low, whisking frequently, until melted and smooth.
Simmer for 5 minutes, then remove from heat.
Stir in the vanilla extract and salt, then allow to cool before transferring.
If needed, add up to ½ cup more water until it reaches your desired consistency.