Sprinkle the sugar evenly into the bottom of a glass dish (a loaf pan works well).
Place the pan in a cold oven and turn the heat on to 400 degrees. As the oven heats, the sugar will begin to melt. When it does, watch it carefully so that it doesn't burn. You want the sugar to melt completely and turn a rich golden-brown.
Remove the pan from the oven and set it on a rack to cool while you prepare the custard. Reduce the oven heat to 375 degrees.
Whisk together the eggs and vanilla.
Add the two cans of condensed milk, and mix thoroughly.
Use one of the empty cans to measure one full can of whole milk. Add this to the mix and whisk thoroughly.
Add cinnamon to taste and whisk.
Pour into the glass dish. You might hear the sugar crackle, but as long as you've let your pan cool a bit, it shouldn't be the glass cracking ;-).
Bake the custard at 375 degrees for 55-60 minutes in a boiling water bath. The custard will still be a bit jiggly at the end of this time, but will set as it cools.
Cool completely, then invert into another dish. Scrape any remaining syrup out of the baking dish over the top of the flan.
This is best if refrigerated completely before serving, but may be eaten at this point.