In a medium saucepan over medium-high heat, bring the water to a boil. Add the couscous in a stream. Stir once. Remove from the heat. Cover and let stand until the couscous is tender, 12 to 15 minutes. Mix in 2 tablespoons of the sugar. Cover tightly and set aside.

Scrub one of the oranges under running water and pat dry. With a vegetable peeler, remove the zest, being careful not to peel away any of the white pith. Finely mince the zest. Set aside.

Squeeze the oranges to obtain about 3/4 cup of juice. Strain through a medium-meshed sieve into a small saucepan. Over medium heat, boil down the juice until it has reduced to about 2 tablespoons, and has the consistency of molten honey, 4 to 5 minutes. Stir in the Cointreau. Set aside.

Peel the mago and cut the flesh away from the seed. Cut half into thin wedges and coarsely dice the rest. Set aside.

In a chilled metal bowl, whip the cream until soft peaks form. Fold in the vanilla and the remaining sugar. Refrigerate 1 cup of the whipped cream for serving. In a medium bowl, combine the rest of the whipped cream with the yogurt and the diced mango. Refrigerate.

Before serving, combine the yogurt-mango mixture with the couscous. Spoon equal amounts into 6 parfait glasses or small bowls. Top each dessert with a generous dollop of the reserved whipped cream and mango wedges. Drizzle with orange-Cointreau sauce, sprinkle with minced orange zest, and serve.