Butter a foil-lined jelly roll pan. In a large, heavy saucepan, bring the sugar, corn syrup and 1/2 cup water to a boil over high heat. Cook until a drop of the syrup hardens in a bowl of ice water or until a candy thermometer inserted in the syrup registers 300 degrees , about 10 minutes.
Remove from the heat. Stir in the cinnamon and food coloring. Stir in the cashews, then pour onto the prepared pan, spreading as thinly as possible. Let cool, about 45 minutes. Break the brittle into pieces.