Prepare an 8x8 baking dish by greasing it heavily or lining with parchment paper. Measure baking soda and 1/2 tbsp citric acid into a small dish.

In a heavy saucepan, combine sugar, corn syrup, and water. Stir gently off heat until moistened.

Place over medium-high heat with a candy thermometer. Boil without stirring until it reaches 300°F (149°C).

Remove from heat immediately. Carefully stir in candy flavoring and food coloring.

Wait 30 seconds, then quickly whisk in the baking soda and 1/2 tbsp citric acid mixture. The sugar will foam up.

Immediately pour into the prepared dish. Let cool for 10-15 minutes until semi-firm.

Score into rectangular strips with a greased knife. Let cool completely (about 2 hours) until hard.

Mix coarse sugar and remaining 1/2 tbsp citric acid in a bowl. Lightly dampen candy strips and roll in the mixture to coat.