Line a 9x13-inch baking dish (see note) with foil, letting about 2 inches of hang over the edge of the dish. Place another sheet of foil crosswise, letting about 2 inches of foil hang over the edge of the dish. (This makes it easy to lift out the marshmallows later on.)

Spray the foil with non-stick cooking spray and add a generous dusting of powdered sugar.

Pour ½ cup cold water into a large mixing bowl or into the bowl of a stand mixer. Sprinkle gelatin on top of the water. Let sit until the gelatin softens and absorbs water, 10-15 minutes.

Meanwhile, make the syrup for the marshmallows. In a medium saucepan, combine sugar, corn syrup, salt and the remaining ½ cup cold water. Cook over medium heat, stirring consistently, until sugar and syrup are completely dissolved. Brush the sides of the pan with a wet pastry brush to prevent sugar granules from burning on the sides of the pan.

Secure a candy thermometer to the side of the pan. Increase heat to medium-high and bring to a boil. Without stirring, boil until the mixture reaches 240°F, about 8 minutes. Remove from heat immediately.

With mixer on low speed, carefully pour hot syrup into gelatin in a slow stream. Gradually increase the mixer speed every so often. Once the mixer is on high-speed, beat until very thick and stiff, about 15 minutes. The mixing bowl should no longer be warm to the touch. Add vanilla extract and bourbon and continue beating until just blended, about 30 seconds.

Spray a rubber spatula with cooking spray and use it to scrape the mixture into the foil-lined pan. Smooth the top with a spatula and sprinkle with powdered sugar. Let rest at room temperature until firm, about 4 hours.

Onto a clean, dry cutting board, sprinkle a generous amount of powdered sugar. Lift the marshmallow slab out of the pan with the foil and turn out onto the the board. Peel off the foil and discard. Sprinkle more powdered sugar on top.

Spray a large, sharp knife with non-stick cooking spray. Trim off edges, then cut marshmallows into 2-inch squares.

Fill a large bowl with powdered sugar and add the cut marshmallows. Toss to coat. Place in an airtight container. Store at room temperature and use within 1 week.