In a food processor, combine ¼ cup Splenda Sweetener with cornstarch and process until sweetener is powder-like in texture. Coat inside of a 9×13” glass baking dish with half of the sweetener-cornstarch mixture and set aside.
In the bowl of a stand mixer, combine gelatin and 1/2 cup water, set aside.
In a saucepan set over medium heat combine remaining Splenda Sweetener and water. Stir gently to moisten all of the sweetener, and cook until sweetener has dissolved and mixture barely begins to bubble, about 4 to 5 minutes.
Begin whisking gelatin mixture on low speed. Slowly add syrup in a steady stream in between the side of the bowl and the whisk attachment, being careful to avoid the whisk itself. Once all the syrup has been incorporated, add peppermint extract and increase speed to high and continue whisking until mixture doubles in volume, about 12 to 14 minutes.
Pour mixture into prepared pan, tilting pan to ensure mixture is evenly distributed. Drop food coloring randomly over surface and use a dinner knife to create a swirl effect. Let stand uncovered at least 8 hours, or overnight.
Dust top of marshmallows with remaining sweetener-cornstarch mixture and turn marshmallow slab out onto a cutting board. Use a sharp, greased knife to cut into individual squares. Store in an airtight container.