Thoroughly grease a 9×13 in baking pan with butter or cooking spray. Mix together ½ cup powdered sugar and ¼ cup cornstarch and sift into the greased pan. Shake to coat.
In the bowl of a stand mixer, whisk together gelatin, vanilla extract, maple extract, and 10 tablespoons water. Gently whisk mixture until well combined and no lumps remain, and the liquid has thickened slightly (about a minute).
In a medium saucepan combine ¾ cup water, corn syrup, granulated sugar, and salt. Whisk to combine and bring to a boil over medium high heat. Cover pot, reduce heat and simmer for about 2 and a half minutes. Remove from heat.
Set your mixer to medium and slowly pour the heated syrup into gelatin mixture. When combined, increase the mixer speed to maximum and beat for 15-20 minutes until the mixture is glossy and makes stiff peaks.
Spread the marshmallow mixture into your prepared pan. Use a wet spatula to smooth the top until it’s even.
Leave the marshmallow mixture uncovered at room temperature for 24 hours to set.
Once the marshmallows have set, combine remaining powdered sugar and cornstarch in a medium sized bowl. Dust your cutting board and the top of the marshmallows with sugar/cornstarch mixture.
Wet a sharp knife and drag it along the edges of the marshmallow pan to loosen the marshmallows. Flip the dish onto your prepared cutting board.
Cut the marshmallows into squares, wetting your knife between cuts to prevent sticking. Toss the cut marshmallow squares in the cornstarch sugar mixture until well coated.
Store marshmallows in a sealed container, layered between wax paper.