Spray a 9x12 baking pan with cooking spray. Spread the oil around the pan with paper towel to make sure there’s a thin layer of oil covering every surface.
Add gelatin to the bowl of a stand mixer. Combine ½ cup cold water with extract of choice and pour mixture over gelatin, whisking gently with a fork. Stir until there are no more large lumps. Set bowl in stand mixer fitted with whisk attachment.
Add ¾ cup water to a 4-quart saucepan. Pour sugar, corn syrup, and salt on top; do not stir. Place pan over medium-high heat and bring to a full boil. To prevent sugar crystals from forming, dip a pastry brush in water and occasionally brush down the sides of the pot. Do not stir the sugar once it has come to a boil or it may crystallize. Boil the syrup until it reaches 247°F to 250°F on a candy thermometer.
Remove pan from heat and, with mixer on medium speed, carefully pour the hot sugar syrup down the side of the bowl into the gelatin. When all the syrup has been added, cover mixer with a clean kitchen towel to prevent marshmallow from splattering. Increase mixer speed to high and whip marshmallow for about 10 min. until thick and glossy. Liquid will be clear and frothy to start then turns white and creamy. The marshmallow increases in volume after about 5 min. After beating for the full 10 min. it will resemble soft-serve ice cream.
With the mixer running on medium, slowly lift the whisk out of the bowl so it spins off as much marshmallow as possible. Using a stiff spatula, scrape the marshmallow mixture into the pan. It’s super sticky so it may not all come out of the bowl; just get as much as you can. Spray your hands lightly with cooking oil and smooth the top of the marshmallow to make it as even as possible. Let the mixture sit uncovered at room temperature for 6 to 24 hours to set.
Once marshmallow has set, combine powdered sugar and cornstarch in a bowl. Sprinkle top of marshmallows with some of the mixture and smooth it with your hand. Flip the block of marshmallow out onto a clean work surface. Use a spatula to pry it out of the pan if necessary. Sprinkle more powdered sugar mixture over the top of the marshmallow block. Use a sharp knife or pizza wheel to cut the marshmallows into squares, or use a cookie cutter to cut shapes. It helps to dip your cutter in water every few cuts to avoid too much sticking. Toss each marshmallow in the powdered sugar mix so all sides are evenly coated.
Marshmallows will keep in an airtight container at room temperature for several weeks.