Heat 1 cup sugar in a saucepan over a medium heat until syrupy and golden. This should take about 8-10 minutes.
Immediately pour the syrup into the bottom of a round baking pan (7-8 inches in diameter) and spread it out to cover the base.
Preheat the oven to 350 F.
In a mixing bowl, whisk together the remaining sugar and cinnamon, then add the eggs and vanilla and whisk well.
Add the milk and whisk until combined.
Pour the mixture through a sieve into the syrup-coated pan. This helps remove any air bubbles in the milk mixture.
Place the pan onto a baking tray, cover it with foil, and fill the tray with water so that roughly half the round baking pan is covered.
Bake for 1 hour.
Remove the baking pan from the oven and set it on the counter to cool to room temperature before transferring it to the fridge to chill for at least 4 hours or overnight.
To remove the flan from the pan, place your serving plate on top of the pan and flip it upside-down. Carefully lift away the pan. The syrupy topping should run down the sides of the flan.