For the cake:
1 cup (2 sticks) unsalted butter
2 cups water
1 cup canola oil
4 cups sugar
1 cup unsweetened cocoa powder
4 cups all-purpose flour
4 large eggs
1 cup buttermilk
1 tbsp. baking soda
½ tsp. salt
1 tbsp. vanilla extract
For the frosting:
4 cups chilled heavy whipping cream
1¼ cups confectioners’ sugar, sifted
For the chocolate topping:
4 oz. bittersweet chocolate, finely chopped
½ cup heavy whipping cream
¼ cup light corn syrup
2 tsp. vanilla extract
To make the cake layers, preheat the oven to 350˚ F. Line three 9-inch round cake pans (or two 10-inch round cake pans) with parchment paper. Butter and flour the inside edges of the pan, shaking out the excess flour.
In a saucepan over medium heat, combine the butter, water and canola oil; heat until the butter is melted. In a large mixing bowl, combine the sugar, cocoa powder, and flour; whisk to blend. Pour the melted butter mixture into the dry ingredients and whisk until smooth. Whisk in the eggs one at a time, then whisk in the buttermilk. Add the baking soda, salt and vanilla to the bowl and whisk just until incorporated. Divide the batter evenly between the prepared pans. Bake for 35-40 minutes or until a toothpick inserted in the center comes out clean.
Let cool in the pans for 15 minutes. Run a knife around the edge of each cake layer and invert onto a wire cooling rack. Allow the cake layers to cool completely before frosting, at least 2 hours.
To make the frosting, add the heavy cream to the bowl of an electric mixer fitted with a whisk attachment. Whip on medium-high speed until soft peaks form. Add the powdered sugar and continue to whip until thoroughly combined and stiff peaks form. Be careful not to over-beat!
To assemble the cake, place one cake layer on a cake platter and spread a layer of the whipped cream frosting over the top. Top with a second cake layer, more frosting (and the third cake layer, if using). Frost the top and sides of the assembled cake. Refrigerate until the frosting has stabilized, at least 1 hour.
To make the chocolate glaze, place the chocolate in a medium bowl. Heat the cream in a small saucepan over medium heat until simmering. Pour the hot cream over the chocolate and let sit 1-2 minutes. Whisk until the mixture is smooth and homogenous. Blend in the corn syrup and vanilla. Pour the glaze into a pitcher or measuring cup and let cool for 10 minutes. (Do not let the glaze cool longer or it may become difficult to pour over the cake.) Slowly pour the glaze over the cake, ensuring that the top is covered and the glaze drips over the sides.
Refrigerate the cake until the glaze is set and the whipped cream frosting is firm, at least 1 hour. Slice with a long, sharp knife, wiping the blade clean between slices.