Grease 9 1/2 x 11" glass casserole dish with butter or Crisco. Mix together corn starch & powdered sugar; sprinkle a few spoonfuls of cornstarch mixture over butter or Crisco and tilt dish around until well coated. Set baking dish aside and save remaining corn starch mixture for later.

Place gelatin and 1/2 cup ice water in the bowl of a stand mixer; set aside.

Mix remaining water, sugar, salt, and corn syrup in saucepan; heat, covered, over medium-high heat for 3-4 minutes.

Clip candy thermometer to side of saucepan; continue heating mixture until it reaches 240 degrees, approximately 6-8 minutes.

Once mixture reaches 240 degrees, remove from heat. Turn on mixer on low (use whisk attachment) and slowly add syrup mixture to mixing bowl.

Turn mixer to high speed and whip for 12-14 minutes, until mixture is quite thick. Add peppermint extract and whip 1 minute more.

Grease a spatula with butter or Crisco and transfer marshmallow mixture to prepared dish. Use a buttered knife to spread it smooth.

 (Optional) Drop red food coloring onto mixture and use buttered knife to pull color through marshmallow mixture to create swirls.

Generously sprinkle the top of marshmallow with remaining cornstarch mixture. Let set overnight.

Invert marshmallows onto a cutting board. Use butter & excess cornstarch mixture to coat sharp knife or pizza cutter blade, then cut marshmallows into 1x1" squares.

Coat cut edges with cornstarch/powdered sugar mixture & wipe off excess. Serve in hot chocolate or package in cellophane bags to give as gifts.