Preheat your oven's broiler. Line a rimmed baking sheet with parchment paper and set aside.
In a small saucepan combine 2 cups water with ½ cup sugar and set over medium-low heat. Cook, stirring occasionally, until sugar has dissolved into water, about 5 minutes.
While that cooks, spread 13 ounces (1 large jar or 2 small jars (which will be 14 ounces, but that's fine)) of marshmallow crème over the parchment paper lined baking tray. This increased surface area will provide more opportunities for toasting!
Broil the marshmallow crème for 2 – 4 minutes, rotating tray as needed, until it is deeply toasted and golden. Alternatively, you can use a kitchen torch to carefully toast the marshmallow.
Remove marshmallow crème from oven and carefully transfer to water and sugar mixture.
Stir to combine and return to heat, brining to a boil and then removing from heat. Steep, stirring occasionally, for 15 – 20 minutes, or until most of the crème has broken down into the simple syrup.
Pour the marshmallow syrup through a fine mesh sieve, discarding whatever solids are left behind.
Skim the top of the toasted marshmallow syrup to remove any foam.
Transfer to a jar, cool to room temperature, and store until ready to use.