With the rack in the middle position, preheat the oven to 180 °C (350 °F). Butter and line a 20-cm (8-inch) springform pan with parchment paper.
In a saucepan, bring water and dates to a boil. Simmer, stirring frequently, for about 3 minutes. Set aside.
In a bowl, combine flour, baking powder, baking soda and salt. Set aside.
In another bowl, cream butter, sugar and vanilla with an electric mixer. Add egg and beat until smooth. At low speed, add dry ingredients alternately with the warm date mixture. Add nuts and combine. Pour into pan. Bake for about 45 minutes or until a toothpick inserted in the centre of the cake comes out clean.
Meanwhile, in a saucepan, bring all ingredients to a boil, stirring constantly. Simmer gently for about 2 minutes.
Spread topping on the warm cake and bake for an additional 5 minutes. Let cool completely before removing from pan.
In a saucepan, bring all ingredients to a boil, stirring constantly. Simmer for about 5 minutes. Let cool. Serve with cake.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.