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Combine the sugar and water in a deep, heavy saucepan. Cover the pan, set it over high heat and boil for 1 minute. Uncover the pan and boil until a dark golden brown caramel forms, 5 to 8 minutes. If sugar crystals splash up on the sides of the pan, brush them down with a natural bristle pastry brush dipped in water.
Remove the pan from the heat and carefully add the heavy cream; stand back—the sauce will hiss and spatter. Return the pan to the heat and simmer the sauce, whisking, until the caramel is completely dissolved in the cream, 2 to 4 minutes. Whisk in the cinnamon. Remove the pan from the heat and let the sauce cool. The sauce can be made several hours ahead.