Custard: Bring the milk to boil in a medium saucepan. While it is heating, place the egg yolks, sugar, & a pinch of salt into a mixing bowl & beat until pale yellow & thick. Add the cornstarch & beat until smooth.
Gradually pour the hot milk into the egg mixture, stirring continuously. Pour back into the saucepan, cook over medium heat, stirring constantly until boiling. Continue to stir vigorously until the custard is smooth, about 45 seconds.
Remove from the heat & continue to beat vigorously for another 30 seconds. Stir in the vanilla & rose water. When cool, place a sheet of plastic wrap directly on the custard so it does not form a layer of skin. Store in the refrigerator until ready to use.
Couscous: Toasted the almonds until golden brown. Chop the almonds & walnuts in a food processor. Add to the dates & raisins then set aside.
Prepare the couscous as directed on the package (should make around 4 cups, cooked). Then stir in the butter & sugar.
When ready to serve, spread the custard on the bottom of a 10-12 inch round serving dish. Cover the custard with half the couscous then scatter the nut mixture evenly over it. Top with the rest of the couscous & sprinkle with the ground pistachios.