Preheat the oven to 300°F, and place six 6-ounce ramekins in a 9x13-inch pan with sides. Fill a pot or kettle with water and bring it to a boil.
In a sauce pan or wide bottom pan over medium-low heat add 1 cup of sugar and leave it to turn a golden color, roughly 10 minutes. You need to watch it closely the hard part is to avoid stirring it. Adjust the heat to how quickly it is browning. Looking for low and slow without it going too dark aka bitter.
Once the caramel is a good color, immediately remove it from the heat and quickly pour it into the ramekins. If you don't get it all poured into the ramekins before it hardens you can put it back on the heat to loosen it up.
In a large mixing bowl, beat the room temperature eggs and remaining ½ cup of sugar until creamy. Add the whole milk and vanilla and stir until well combined.
Divide the custard evenly among the six ramekins, pouring on top of the caramel. Cover each one with foil and place in the 9x13-inch pan. Fill the pan with boiling water halfway up the sides of the ramekins.
Bake at 300°F for 50-60 minutes, or until it jiggles when you lightly shake the ramekin. The flan is cooked when it has an internal temperature of 175°F in the center.
Take the ramekins out of the water-filled baking dish and set aside to cool to room temperature. Once completely cooled, cover with foil and place in the fridge for a minimum of 6 hours, better if overnight is an option.
To serve, take a butter knife and run it around the inside of the ramekin. Place a plate on top of the ramekin and invert it, then remove the ramekin and hope that the caramel oozes all over the flan. Serve immediately.