- Preheat the oven to 350°F (180°C). Lightly grease three 8-inch (20 cm) cake pans. Line the bottoms with parchment paper and dust the sides with flour, tapping out any excess.
- Pour the hot coffee over the chocolate in a small bowl. Let this sit for a minute and then whisk until the mixture is smooth. You can work with this while still warm.
- Sift the flour, sugar, cocoa powder, baking soda, baking powder and salt into a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment. Stir in the brown sugar. Add the diced butter and oil, working them in using electric beaters or the mixer on medium speed, until large pieces of butter are no longer visible, about 3 minutes.
- Whisk the milk, eggs, egg yolk and vanilla together and add this all at once to the flour mixture and mix on low speed until combined and then increase the speed to medium, beating until smooth and thick, about a minute. Add the melted chocolate and mix again on medium speed until smooth, about 2 minutes. Divide the batter between the pans and spread to level them, giving the pans a tap to knock out any air bubbles. Bake the cakes for 30 to 35 minutes, until a tester inserted in the centre of a cake comes out clean. Cool the cakes in their pans on a rack for 15 minutes, then turn them out onto the rack to cool completely.
- For the creamy filling, whisk ½ cup (125 mL) each of the milk and water with the cocoa powder and 1/3 cup (70 g) of the sugar in a medium saucepan and then bring it to a simmer over medium heat. While the milk heats, whisk the remaining ¼ cup (60 mL) each of milk and water with the remaining ⅓ cup (70 g) sugar, egg yolks and cornstarch in a medium bowl. In a separate, larger bowl, place the chocolate and butter.
- Once the milk comes up to a simmer, slowly pour this into egg mixture while whisking and then add the combined mixture back to the pot. Continue whisking over medium heat until the filling thickens, looks glossy and just begins to bubble and then scrape this into the bowl with the butter and chocolate, whisking until both are melted. Stir in the vanilla. Place a cover directly on the surface of the filling, cool for an hour on the counter and then chill for at least 4 hours before using.
- For the ganache topping, place the chopped chocolate in a mixing bowl. Heat the cream and butter over medium heat until it comes to a simmer, pour it over the chocolate and let this sit for a minute. Slowly whisk or stir the ganache until it is smooth. Let this cool for 30 minutes and then chill for another 30 minutes until spreadable (this can also be made ahead, chilled and reheated over low heat to soften).
- To assemble, place a cake layer on a cake stand or cake wheel. Spread a generous quarter of the creamy filling evenly over the cake and top with a second cake. Spread another quarter of the filling over this cake and then top with the third cake layer. Spread most of the remaining filling over the top, making it as level as possible, and use any remaining filling to spread a sheer coating on the side of the cake. If the ganache is still too fluid to set, chill the cake until ready.
- Spread the ganache over the top of the cake and coax it over the top edge. Spread this overhanging ganache over the side of the cake (it does not have to fully coat the side of the cake). Spin the cake as you use an offset spatula to coax the ganache above the top edge of the cake, creating a ruffled seam about a 1/2-inch (12 mm) above the top of the cake, while the top and sides of the cake are smooth. If decorating with gold flake, use tweezers to place a few flakes on the top of the cake and chill, uncovered, until ready to serve.
The cake will keep, refrigerated, for up to 4 days.