Pour the sugar into a skillet over medium heat. Stir the sugar continually until it begins to brown and clump together. Keep stirring until the sugar completely melts and dissolves into a golden-brown syrup. Remove the syrup from the heat as soon as it has dissolves as it can burn.
Working quickly, carefully pour the caramel into a 7-inch round pan or a special flan pan (flanera) as shown in the photos in the blog post. Pour the caramel evenly into the bottom of the pan so the entire bottom is coated. If you need to, use an oven mitt or potholders to move the pan from side to side to get the caramel evenly distributed. If the caramel hardens too quickly, you can carefully reheat the pan by placing it directly on the stove or on the skillet for a few seconds.
Set the caramel-coated pan aside to cool and harden.
In a large mixing bowl, add the eggs, sweetened condensed milk, whole milk and vanilla extract. Whisk together until everything is fully combined, but try not to overwhisk so that there aren't too many air bubbles in the mixture.
Pour the custard mixture into the caramel-coated pan. Cover the top of the pan with the lid (if using a flanera) or tightly with aluminum foil.
Place the flan pan in a large baking dish and pour enough water in the dish until it reaches about halfway up the pan. Carefully place the baking dish in the oven and bake for 1 hour and 15 minutes.
Carefully remove the dish from the oven, and then lift the flan pan out of the water bath. Remove the lid or aluminum foil and let it cool to room temperature. The flan may still be a little jiggly, but that's okay - it will continue to cook with the residual heat.
Once it's cooled, cover with the lid or aluminum foil and refrigerate for 4 hours or overnight.
To serve, remove the lid or aluminum foil and slide a knife along the edges of the pan to loosen it up. Place a plate or platter on top of the flan, grab a hold of both the plate and the pan and quickly and securely flip it over so that the flan is now upside down. Carefully lift off the flan pan. Your flan should be sitting in the caramel on the plate, ready to enjoy!
Carefully lift the flan pan up. The flan should be on the plate - if it's not, place the pan back down and let it sit for a few minutes. Don't freak out! It will loosen up and slide out. You could try tapping the sides of the pan a bit or even giving the sides a squeeze.
Slice the flan and serve with the rich caramel sauce!