Whisk together the yolks, condensed milk, and evaporated milk; set aside. Melt margarine in a small saucepan over medium heat. Stir in the brown sugar; keep stirring until the brown sugar has dissolved, and is smooth. Evenly spread the sugar over the bottom of a pie plate, then pour in the custard. Cover with foil. (See Cook's Note.)

Fill a large pot approximately 3/4 full with water and bring to a simmer on the stove. Place the pie plate over the simmering water and cook for 65 to 70 minutes, until flan has set. Remove flan from the boiling water and allow to cool for at least 30 minutes.

To serve, loosen the flan from the pie plate by running a knife around the edges; invert onto a serving plate. Serve at room temperature or cold.