01 - Line a large baking sheet with parchment paper and lightly dust with powdered sugar.
02 - In a heavy saucepan, combine granulated sugar, corn syrup, water, and cream of tartar. Stir over medium heat until sugar is fully dissolved.
03 - Increase heat and bring the mixture to a boil. Without stirring, cook until it reaches 290°F (soft-crack stage) as measured by a candy thermometer.
04 - Remove saucepan from heat, add peppermint extract, and stir immediately to incorporate.
05 - Pour half of the hot candy onto a greased marble slab or heatproof surface. Add red gel food coloring to this portion; leave the other half plain.
06 - Using oiled spatulas or heatproof gloves, fold and pull each portion separately until cool enough to handle but still pliable, approximately 2 to 3 minutes.
07 - Pull each portion into a long rope, then twist the red and white ropes together to create the classic peppermint swirl.
08 - Using oiled scissors, cut the twisted ropes into 2-inch pieces and arrange on the prepared baking sheet to cool completely.
09 - Transfer cooled candies into an airtight container and store at room temperature.