Prepare a 7x11 pan (see notes). Lightly coat the pan with a flavorless oil, like canola, and line with two sheets of parchment. Make sure one of the sheets of paper is long enough to fold over the rest of the pan. Grease the parchment paper or liberally dust with the powdered sugar mixture. Set aside.
Place the gelatin and 4 fl oz of cold hibiscus tea in the bowl of a stand mixer. Use a fork to mix well and break up any lumps. Set aside to bloom.
In a saucepan, combine the corn syrup, sugar, and water. Stir just to combine. Cover and place over medium- high heat for 5 minutes.
Remove the lid and attach a candy thermometer to the pan. Continue to cook, without stirring, until the syrup hits 240°F. This will take roughly 8- 12 minutes.
Once the syrup is hot enough, turn the mixer (fitted with a whisk) on medium- low. Slowly and carefully drizzle the hot syrup into the bowl, in between the bowl and the whisk. Avoid letting the syrup touch the whisk by aiming for the side of the bowl. Add the salt.
Bump the speed up to medium- high and whip for 5 minutes. Turn off the mixer and add the liquor and gel color. Turn the machine back on at a low speed, slowly increasing the speed to medium- high. Whip for another 3- 5 minutes.
Oil a spatula and a small offset spatula. Use them to add the marshmallow goo to the prepared pan, smoothing the top into an even layer.
Cover the top of the marshmallow goo with the long piece of greased parchment paper or dust with the powdered sugar mix and then a piece of parchment paper. Cover the pan with plastic wrap and/ or foil and let it set for at least 3 hours, but I always do overnight.
Oil a large knife, bench scraper, or cake smoother. Cover a cutting board with a generous layer of the powdered sugar mix. Carefully remove the marshmallow slab from the pan. Cut into cubes. Toss the marshmallow cubes, 1- 5 at a time, in the bowl of powdered sugar mix. Then place them in a sieve and tap off the excess powder.
Store the marshmallows in an airtight container, layered between wax paper, at room temperature for up to 3 or 4 weeks.