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 tablespoon baking soda
½ teaspoon salt
1 tablespoon vanilla extract
For the frosting:
4 cups chilled heavy whipping cream
1¼ cups confectioners’ sugar, sifted
For the chocolate glaze:
4 ounces bittersweet chocolate, finely chopped
½ cup heavy whipping cream
¼ cup light corn syrup
2 teaspoons vanilla extract
Start by making the cakes. Heat oven to 350º. Butter three 9-inch or two 10-inch cake pans and dust with cocoa powder, shaking out the excess. Line the bottoms with parchment.
Combine the butter, water and oil in a saucepan set over medium heat. Cook until the butter is melted.
In the bowl of an electric mixer, whisk together the sugar, cocoa powder and flour. Pour the butter mixture into the flour mixture and mix on low speed until smooth. Increase the speed to medium and add the eggs one at a time, mixing until each is incorporated, then mix in the buttermilk. Add the baking soda, salt and vanilla and mix until just combined.
Divide the batter between the prepared pans. Bake, rotating pans halfway through, for 35-40 minutes or until a toothpick inserted in the center comes out clean.
Allow the cakes to cool in the pans for 15 minutes, then invert onto a wire rack to cool completely, about 2 hours.
To make the frosting, add the heavy cream to the bowl of an electric mixer fitted with the whisk attachment. Whip on medium-high speed until soft peaks form. Add the powdered sugar and continue mixing until stiff peaks form, taking care not to over whip.
To assemble the cake, place one layer on a cake platter. Spread a layer of frosting over the top, then repeat with the next layer. Frost the top and sides of the assembled cake, then refrigerate for at least an hour to let the frosting stabilize.
To make the glaze, place the chocolate in a medium bowl. Heat the cream in a small saucepan over medium heat until just simmering. Pour the cream over the chocolate and let sit for about 2 minutes. Whisk the chocolate and cream until smooth, then stir in the corn syrup and vanilla. Transfer to a liquid measuring cup and allow to sit about 10 minutes. Don’t let it cool too long or it won’t pour over the cake easily. Slowly pour the glaze over the top of the cake, making sure to cover the entire top and allowing some to drip partially down the sides.
Refrigerate until the glaze is set, about an hour. Use a long, sharp knife to cut the cake into slices and serve.