1. Combine sugar, water, and lemon juice in small saucepan; bring to boil over high heat and boil until temperature reaches 238 degree F (soft-ball stage) on a candy thermometer, 11 to 14 minutes.
2. Meanwhile, for meringue, beat egg whites in large bowl at medium speed of electric mixer to soft peaks. Increase speed to high; gradually beat in hot syrup in a slow steady stream. Continue to beat until meringue cools to room temperature, about 6 minutes.
3. Scrape seeds from vanilla bean into large bowl; discard bean. Whisk in mascarpone, cream, and cinnamon to combine. Gently fold in meringue.
4. Spoon mousse into twelve 5-ounce paper or plastic cups. Tap gently to remove air bubbles. Freeze mousse 4 hours or until solid. Peel cup away from each mousse. Refrigerate 30 minutes before serving. Makes 12 servings.