Spread caramel from flan box on bottom of 4-cup mold.
Add pumpkin purée, milk and cinnamon to small saucepan over medium-high heat, whisking to combine. Whisk in remaining contents of flan box; bring to boil, whisking occasionally until smooth and combined. Pour pumpkin mixture in prepared mold; cover with plastic wrap. Transfer mold to refrigerator. Chill until cold and firm, at least 4 hours, or up to 24 hours.
To unmold, run a thin knife around edges of mold. Invert onto serving plate. To serve, cut into 10 wedges. Serve chilled.