Line a baking sheet with parchment paper, if you're not using a mold. Set aside.
In a heavy saucepan, combine sugar, corn syrup, and water. Stir well, bring to a boil on medium or medium-high heat. Affix candy thermometer to pan.
Boil mixture, stirring occasionally, until it reaches 310 degrees.
Remove from heat, quickly stir in food coloring, flavoring, and citric acid. Stir mixture until it stops boiling, and most of the “bubbles” disappear – this will minimize texture in your finished candies.
If using a Mold:
Carefully pour molten candy into into mold cavities.
Allow to fully cool and harden.
If using a Cookie Sheet:
Carefully pour molten candy into prepared cookie sheet.Allow to cool until still very warm to the touch, slightly malleable, and firm (not brittle).
Turn slab of candy out onto a cutting board, taking care not to stretch it. Using a large knife, score candy slab into rows about 1″ apart. Turn slab 90 degrees, and score again – ½″ for long skinny candies, 1″ for square ones.
Working quickly, cut all the way through the candy slab along the score lines, separating candies into rows, then individual candies. You will need to have all of your cutting done while the candy is still fairly warm and malleable, or it could shatter when you attempt to cut it.
Notes
Hard candy is very hydrophilic – it pulls moisture out of the air, and absorbs it. This will result in sticky or “sweaty” candy – you’ll want to either wrap them individually, or keep them sealed in an airtight container