Beat together the egg yolks, condensed milk and evaporated milk; set aside.
Melt margarine in a small saucepan over medium heat. Stir in the dark brown soft sugar; keep stirring until the dark brown soft sugar has dissolved and is smooth.
Evenly spread the sugar over the bottom of a pie dish, then pour in the egg yolk-condensed milk mixture. Cover with foil. (See footnote).
Fill a large pot approximately 3/4 full with water and bring to a simmer on the hob. Place the pie dish 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 dish by running a knife around the edges; invert onto a serving plate. Serve at room temperature or cold.
Since this dessert is cooked on the hob and not in the oven, it's important to have a pie dish with handles and an appropriately sized, large tin. The two must fit together well, securely holding the pie dish over the simmering water while the dessert cooks.