Melt sugar in a small saucepan over medium low heat, stirring occasionally. When sugar has melted and is a deep amber color, quickly pour into flan molds and set aside.
Put a kettle of water on to boil for the water bath. Scald milk in a small saucepan over medium heat, then stir in the dulce de leche until dissolved and remove fro heat. While milk is heating, whisk together the eggs, yolks, and vanilla unti pale yellow.
Very slowly drizzle the milk and dulce de leche into the eggs, whisking constantly. As the eggs become tempered, increase the speed of pouring. When all the milk mixture is incorporated, pour the custard into the molds over the caramel. Place the molds in a high sided baking pan and carefully pour hot water around them, coming to an inch from the tops. BAke flans for 30-40 minutes, until set in the center. Remove from water bath and allow to cool on a wire rack for 20 minutes, then chill. To serve, place each mold in boiling water for 30 seconds, then run a knife around the inside to loosen. Invert on a serving plate and allow caramel to run over the flans.