- 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.