1Heat oven to 350°F (325°F for dark or nonstick pan). Grease or spray bottom only of 13x9-inch pan. Make cake mix as directed on box--except add cinnamon to cake mix. Pour into pan.
2Bake as directed on box for 13x9-inch pan. Cool completely, about 1 hour.
3In 2-quart heavy saucepan, heat granulated sugar over medium-low heat 6 to 8 minutes, stirring constantly with wooden spoon and watching carefully, until sugar begins to melt. As sugar begins to melt, stir with wooden spoon until sugar is melted and golden brown. (Sugar becomes very hot and could melt a plastic spoon.) Remove from heat; carefully stir in butter and 1 tablespoon of the milk (mixture will become lumpy). Return to medium-low heat, stirring constantly, until mixture is smooth. Cool 5 minutes.
4Add powdered sugar to caramel mixture; beat with electric mixer on low speed until well combined. Add 3 tablespoons milk, mixing until frosting is glossy and spreadable. If necessary, add up to 1 tablespoon more milk, 1 teaspoon at a time. Spread frosting over cake. Store loosely covered.