For this ever-so-satisfying chocolate birthday cake, use the best bittersweet chocolate you can find. To keep the cake dairy-free, read the ingredient list carefully to make sure it doesn't contain any milk solids.
Line the base of a 9-inch (23-cm) round springform pan with parchment paper.
Place water and chocolate in a small, heavy pot over medium-low heat and stir until chocolate is almost fully melted. Remove from heat and continue to stir until chocolate is melted and mixture is well combined. Stir in oil and vanilla and set aside.
Combine flour, cocoa, sugar, baking soda and salt in a bowl and stir with a fork until uniform. Add liquid mixture to dry ingredients and stir until well combined. Scrape batter into prepared pan.
Pour vinegar on top of batter and use a fork to incorporate it into the batter as quickly as possible. There will be pale swirls in the batter from the baking soda and vinegar reacting. Stir just until vinegar is evenly distributed. Bake for 35 to 40 minutes or until a cake tester comes out clean.
Let cake cool fully in pan, then run a knife around the edge to loosen it from the sides. Remove sides of pan, carefully invert cake onto a plate, remove bottom of pan and peel off parchment paper. Use another plate to turn cake right-side up.
Place cake on serving plate and make glaze. Place chocolate, water and corn syrup in a small, heavy pot over medium-low heat and stir until chocolate is almost fully melted. Remove from heat and continue to stir until chocolate is melted and mixture is shiny. Allow to cool for 5 minutes or until it has thickened slightly, and then pour glaze evenly over top of cake.
Per serving: 360 calories, 21 g total fat, 6 g saturated fat, 6.5 g monounsaturated fat, 3 g polyunsaturated fat, 46 g carbohydrates, 4 g protein, 0 mg cholesterol, 2 g fibre, 40 mg calcium, 25 mg magnesium, 250 mg sodium, 280 mg potassium, 1.9 mg vitamin E, 3 mg iron, 1.7 RE vitamin A, 0 mg vitamin C