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 then keep stirring until it 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 note at bottom.)
Fill a large saucepan 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.
Since this flan is cooked on the stove and not in the oven it is important to have a pie plate with handles and an appropriately sized large saucepan. The two must fit together well and securely holding the pie plate over the simmering water while the flan cooks.