In a toaster oven, lay marshmallows on a baking sheet and toast until golden brown. Flip the marshmallows to toast all sides. Alternatively, thread marshmallows onto a skewer and use a kitchen torch or gas stove to brown the marshmallows.

While the marshmallows are toasting, in a medium saucepan, combine sugar and water over medium heat. Bring mixture to a gentle simmer, stirring occasionally until the sugar dissolves.

Stir in the roasted marshmallows, whisk until combined. Simmer for 5 minutes or until the marshmallows have dissolved.

Remove from the heat and stir in vanilla extract. Let the syrup cool to room temperature. Strain the mixture using a fine mesh strainer, then transfer liquid to a clean container.

Store the simple syrup in an airtight container in the refrigerator for up to three weeks.