CHOCOLATE BROWNIE CAKE

1. Preheat the oven to 350° / 180°. Grease and line three six inch cake pans with parchment paper.
2. In a small saucepan over medium heat, heat the butter and chocolate, stirring constantly, until melted and combined. Set aside to cool.
3. In a small bowl, whisk together the flour, cocoa powder, baking powder, and salt.
4. In a large bowl, whisk together the sugar, dark brown sugar, eggs, vanilla, and oil. Add the cooled chocolate mixture, and mix to combine. Add the dry ingredients, and gently fold to combine.
5. Divide the batter between the three cake pans, and bang once or twice on the counter to help settle.
6. Bake for 25 to 30 minutes, checking for doneness after 20 minutes, until a the top of the cakes are set, and skewer inserted in the middle of the cake comes out with a few crumbs attached.
7. Cool in the tins for 10 minutes, then turn out onto a wire rack and allow to cool completely. If you are making ahead, once cool, wrap tightly in plastic wrap and store in the fridge until you are ready to assemble the cake.

CHOCOLATE HAZELNUT FILLING

1. Preheat the oven to 350˚f / 180˚c. Place the hazelnuts on a baking sheet. Roast in the oven, shaking occasionally, for 8-10 mins, checking often, until the are golden brown. Allow to cool completely, then rub between your hands to remove the skins. Do not worry about any stubborn skins. Transfer to the work bowl of a food processor, and pulse briefly to chop. 
2. Add the melted dark chocolate, sugar, and salt. Pulse to combine, then mix on high speed for at least five minutes. Check, and give more time mixing if needed. Transfer to a container and store at room temperature until you are ready to use. 

MOCHA GERMAN BUTTERCREAM

1. In a medium saucepan, heat the milk to just shy of a simmer.  Remove from the heat, add the coffee, stir well, and steep for 20 minutes. Strain through a very fine mesh sieve, and re-weigh the infused milk and top up to 375g. Wipe out the saucepan, and return to the milk mixture to the heat. Bring to just shy of a simmer. Meanwhile, In a bowl, whisk together the vanilla, corn starch, sugar, egg, egg yolk and salt.
2. Using one hand to whisk constantly, pour half of the hot milk mixture into the egg mixture. This helps to temper the eggs and stop them from scrambling. Whisk until incorporated, and then pour the whole lot back into the saucepan.
3. Heat the milk and egg mixture over medium heat, whisking constantly, until it begins to bubble. It will thicken quickly. Once it has thickened, cook for one minute, then remove from the heat. Pour into a shallow container or bowl of a stand mixer and press some plastic wrap over the surface to avoid a skin from forming. Refrigerate until cold - at least four hours, preferably overnight. If you need to speed this process up, you can place the pastry cream in a bowl, then place the bowl in an ice bath. Stir frequently.
4. Fit your mixer with the whisk attachment, and place the pastry cream in the bowl. Whip the mixture on medium until creamy and lump-free. Begin adding the butter, a few cubes at a time, until fully incorporated. It may look curdled at some point but just keep whipping - it will come together! Whip for a further 2 minutes.
5. Remove 250g of the coffee buttercream, and transfer to a piping bag (do not snip the end off just yet). Add the cooled chocolate to the remaining coffee buttercream left in the mixer, and mix to combine.

ASSEMBLY

1. If the cakes domed in the oven, level them a little with a sharp bread knife. Transfer some of the mocha buttercream to a piping bag fitted with a medium round tip - this will be used to make a buttercream dam to hold in the chocolate filling between layers.
2. Secure one of the layers of cake to a cake turntable using a little buttercream. Add about half a cup of mocha buttercream onto the first layer of cake, and smooth using an offset spatula. Create a buttercream dam using the buttercream in the piping bag by piping a ring of frosting around the outside edge of the first layer. Fill in the ring with approximately ⅓ cup of the hazelnut filling. Place the second layer of cake onto the first, pressing very lightly to secure, and sealing the joins with a thin layer of buttercream. Repeat the layering process - add half a cup of buttercream, make a dam, fill with ⅓ cup hazelnut filling, then top with the third layer of cake - I like to put this one upside down to ensure that the top of the cake is flat.
3. Crumb coat the cake - to do this, apply a thin layer of buttercream over the surface of the cake, and smooth with a bench scraper or icing smoother. Refrigerate the cake for 30-45 minutes, until the crumb coat has set.
4. To ice without stripes (see notes): Cover the cake with a second, thicker layer of mocha buttercream. Smooth. Decorate as desired.
5. To ice with stripes: (see this highlight for more details / photos) Apply a thick layer of buttercream, and smooth completely, ensuring that there are no holes and it is completely smooth. Chill 15 minutes. Using a warmed icing comb (run under hot water then dry), go around the cake, creating grooves in your buttercream. Make sure that the grooves are sharp edged and deep - you may have to do a few passes to ensure this.
6. Freeze the cake for 30 minutes, then, using the reserved coffee buttercream, snip a small hole off the end of the piping bag, and pipe coffee buttercream into the grooves. Do not be afraid to overfill slightly. Once you have filled all of the grooves, smooth the cake using a warmed cake scraper, wiping between passes. Chill the cake for 10-20 minutes.
7. Place the remainder of the buttercream in piping bags fitted with french star tips (ateco #866 is my favourite) - I did one for the mocha buttercream, one for the coffee buttercream, and one for the excess scraped off from smoothing off the stripes, which was marbled.
8. Add gold leaf to the cake where desired, then pipe blobs of buttercream randomly over the surface of the cake, as desired.
9. Chill cake until ready to serve, then bring out of the fridge to come to room temperature about an hour before you serve. Store leftovers in an airtight container in the fridge.