Add gelatin and 1/2 cup cold water to the bowl of a stand mixer.
In a small saucepan, whisk together 1/2 cup water, sugar, corn syrup, and salt. Place on med-high heat, cover, and cook 3-4 minutes. Uncover and add candy thermometer to side of pan. cook until it reaches 240 degrees F. Immediately remove from heat.
Start mixer on low speed. Slowly pour hot sugar mixture into the gelatin. Once all the sugar syrup is added, turn mixer to high until marshmallow mixture is very thick and slightly warm, 12-15 minutes.
Add vanilla and peppermint extract. Whisk 1 minute more.
While mixer is going, stir together cornstarch and powdered sugar. Grease a 9-inch x 13-inch pan with non-stick spray. Coat the inside of with cornstarch mixture. Pour any excess cornstarch into a bowl. (you'll need it)
Pour marshmallow mixture into prepared pan. Grease the back of a spoon and use to spread into an even layer.
Drip red flooding coloring over the top. Run a knife (coated in cooking spray) through the food coloring to swirl into the marshmallows.
Dust top with enough cornstarch mixture to lightly coat. Let sit uncovered for at least 4 hours to overnight.
Turn marshmallows out onto a cutting board and cut into 1-inch cubes. (I like to use a pizza wheel dusted in powdered sugar.) Dust all sides of each marshmallow with more cornstarch mixture (make more if needed).
Notes
These marshmallows can be stored in an airtight container for up to 3 weeks.