For the Cake: With the rack in the middle position, preheat the oven to 180°C (350°F). Butter and flour a 20-cm (8-inch) springform pan. Line the bottom with parchment paper.
In a saucepan, bring the water, dates, butter and baking soda to a boil. Reduce the heat and simmer, stirring frequently, for about 3 minutes. Transfer to a bowl and let cool slightly. Cover and refrigerate until lukewarm, about 30 minutes.
In a bowl, combine the flour, couscous and baking powder. Set aside.
In another bowl, beat the eggs with the sugar using an electric mixer until the mixture turns pale, triples in volume and falls from the beaters in a ribbon, about 10 minutes. Using a spatula, gently fold in the dry ingredients then the date mixture.
Pour into the springform pan and bake until a toothpick inserted in the centre comes out clean, about 65 minutes.
Let cool. Run the blade of a thin knife around the inside of the pan. Unmould and let cool on a rack.
For the Icing: In a bowl, beat the sugar and milk until smooth. Drizzle over the cooled cake.