Spray an 11-inch round baking pan with nonstick cooking spray. Combine sugar and water in a medium saucepan. While stirring to dissolve the sugar, bring to a full boil over medium-high heat. Reduce heat to medium low; let cook 20 to 25 minutes, until sugar turns golden brown and caramelizes. Do not stir. Quickly pour caramelized sugar mixture into baking pan, tilting to evenly cover the bottom of the pan with the glaze. Set aside.

Preheat oven to 375 degrees.

In a medium bowl whisk together 1 1/2 cups milk, egg yolks, condensed milk, vanilla, and cinnamon until well blended. Pour over the caramelized sugar mixture in the pan.

In a medium bowl, combine cake mix, 1/2 cup milk, and eggs, and mix well. Spoon batter evenly over the custard mixture.

Bake 30-35 minutes or until cake springs back when touched lightly in center. Cool 20 minutes. Loosen edge with knife. Invert dessert onto a large heat-resistent serving plate.

Serve warm or cold. Store leftover dessert covered in the refrigerator.