With the rack in the middle position, preheat the oven to 180 ˚C (350 F).
Line two 23-cm (9-inch) springform pans with parchment paper, butter and flour thoroughly.
In a bowl, sift together the flour, cocoa powder, and baking powder. Set aside.
In another bowl, with an electric mixer on high speed, beat the eggs, cream of tartar and sugar until the mixture is light and fluffy and doubles in volume, about 8 minutes. Add the oil, buttermilk and vanilla. Reduce the speed of the electric mixer. Add the dry ingredients, combining quickly.
Pour into the prepared pans. Bake for about 1 hour or until a toothpick inserted in the centre of the cake comes out clean. Cool on a wire rack. Cut each cake in half horizontally.
In a saucepan, bring the sugar and water to a boil.
In a bowl, beat all the ingredients until smooth.
Brush each cake layer with syrup. Layer with frosting. Finish by frosting the cake evenly.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.