Prepare the coating:
In a medium bowl combine the powdered sugar and cornstarch. Cover and set aside.
Prepare the pan:
Spray an 8”x8” square pan with PAM spray and sift the marshmallow coating to bottom of the pan. You are looking for an even coat. Set the pan aside.
Prepare the bloom:
To the bowl of a stand mixer fitted with a whisk attachment, combine the gelatin, lime juice, water, and lime zest. Stir with a fork lightly to combine and prevent the gelatin from clumping to the bottom of the bowl. Leave in the mixer while you prepare the sugar syrup.
Prepare the sugar syrup:
In a medium stainless steel pot combine the light corn syrup, granulated sugar, dark rum, water, mint leaves, vanilla bean paste, and salt. Bring to a boil over medium high heat. Cover the pan with a lid and continuing boiling, covered, for 5 minutes. Covering the pan allows steam to form which melts any sugar which may have crystalized. After five minutes, insert a candy thermometer and bring up the temperature of the syrup to 240° Fahrenheit.
Once the syrup has reached 240° Fahrenheit, remove pan from heat and allow the temperature of the syrup to reduce to 210° Fahrenheit. This can take anywhere from 7-10 minutes. You can help it along by partially submerging the pan in a bowl of ice water and stirring the syrup.
Once the syrup has reached 210° Fahrenheit, fish out the mint leaves with a fork. Discard the mint leaves. Carefully pour the syrup (be careful – It’s STILL HOT!) into the mixing bowl with the bloomed gelatin. Start your mixer, on low at first as you add the syrup, and then increase the speed to high. Allow the mixture to double in volume.
Turn off the mixer and using a silicone spatula very lightly sprayed with PAM, pour the mixture into the prepared pan. Smack the pan a few times against your counter to remove air bubbles and sift some of your prepared coating over the top of the marshmallow mixture.
Cover with plastic wrap and refrigerate or leaave at room temperature to set for at least 4 hours.
To cut marshmallows:
Remove marshmallow pan from the fridge. Liberally sift your work surface with the marshmallow coating. I mean, be liberal with it. Don’t skimp on it. Remove the plastic wrap from the pan.
Use a knife (or your fingers) to loosen the marshmallow from the pan and pull it out. Place onto the powdered surface and then flip the marshmallow square to coat the other side.
Using a large chef’s knife (or the largest knife in your kitchen), cut 8, 1” vertical strips. Periodically clean your knife because marshmallows stick! Now with your knife make 1” increment cuts horizontally. You should now have a crap load of marshmallows squares - Or rectangles if you have linear issues.
As you cut the marshmallows, toss them into the marshmallow coating to prevent sticking. Store them in an airtight container or a large ziplock back. They last forever…Really – Weeks and weeks.