Once it reached 250 degrees, turn the stand mixer on low and remove the syrup pot from the heat. Gradually add the syrup to the stand mixer in a single stream of syrup. When all the syrup is added, increase the mixer's speed to medium and beat the mixture until it's fluffy, thick, and shiny, about 8-10 minutes.
If you're making the black currant & lavender marshmallows, add the powdered mixture (except the tablespoon you set aside) during the last minute or two of mixing and continue mixing until they're evenly distributed throughout the mixture and it's and even shade of lavender.
If you're making the vanilla bean & rose marshmallows, add the vanilla bean paste, vanilla extract, & rose water during the last minute or two of mixing and continue mixing until they're evenly distributed and there aren't any streaks of tan from the vanilla extract.
Peel the parchment paper off the top of the marshmallow sheet and turn the pan onto a clean flat surface generously dusted with powdered sugar. Lightly dust the top of the sheet with powdered sugar before cutting your marshmallows into roughly 2 x 2 inch cubes using a lightly greased pizza wheel.
Toss them gently, a few at a time, in the bowl of powdered sugar, shaking excess powdered sugar off of them above the bowl before setting them aside. You can store them in an air-tight container for up to 2 weeks, just make sure not the expose them to free-flowing air for an extended period of time otherwise they will get stale and the texture won't be as nice and fluffy.
Adapted from Food & Wine