Remove all unpopped kernels from popped corn. Salt popcorn, if you like. Place the popcorn in a greased 17x12x2-inch baking pan. Keep warm in a 300 degree F oven while making syrup.
For syrup mixture, butter the sides of a heavy 1-1/2-quart saucepan. In saucepan, combine sugar, corn syrup, and water. Cook over medium-high heat until mixture boils, stirring to dissolve sugar (about 8 minutes).
Clip a candy thermometer to side of pan. Reduce heat to medium; continue boiling at a moderate, steady rate, until thermometer registers 300 degree F (hard-crack stage), stirring occasionally (about 25 to 30 minutes).
Remove from heat. Remove thermometer. Carefully stir in vanilla and food coloring, if you like. Pour in a thin stream over hot popcorn; stir gently to coat. Cool. Using gloved hands or a wooden spoon, break into pieces. Store tightly covered or in plastic bags. Makes 16 (1-cup) servings.