Making the cake
Preheat the oven to 180ºC.
Grease a 33 cm x 23 cm traybake tin and line with parchment paper overlapping the edges.
Sift flour, cocoa powder, baking powder and bicarbonate of soda into a bowl.
Add eggs, vegetable oil, sour cream, coffee, salt, light brown sugar and the flour mixture into the Appliance bowl.
Fit the Whisk Tool.
Whisk for 3 minutes on speed 3, until combined.
Tip: Start on speed Min and gradually increase speed.
Transfer the mixture into the tin.
Bake for 25 minutes at 180ºC, until an inserted skewer comes out clean.
Keep the oven turned on at 180°C.
Remove the cake from the oven and let cool in the tin for 20 minutes.
Then lift the overhanging parchment paper and transfer the cake to a wire rack to cool completely.

Line a baking tray with parchment paper.
Add hazelnuts onto the baking tray and spread out evenly.
Bake for 5 minutes at 180ºC, until golden brown.
Remove from the oven and let cool.

Making the coffee buttercream
Add coffee granules and hot water into a bowl and stir until the coffee has dissolved.
Clean the Appliance bowl and fit the K-Beater.
Add butter, icing sugar and hot coffee into the Appliance bowl.
Mix for 2 minutes on speed 1 and then scrape the bowl.
Tip: Start on speed Min and gradually increase speed.
Mix for 2 minutes on speed 1.
Tip: Add a few tablespoons of milk if the mixture is too thick.
Place the cake on a serving board.
Spread the coffee buttercream evenly over the top of the cake.

Add melted chocolate and melted butter into a clean bowl.
Mix until combined.
Using a fork, drizzle the melted chocolate over the cake.
Sprinkle over the toasted hazelnuts.

Serve.

Substitutions
Hot coffee can be replaced with hot water.
Dark chocolate can be replaced with milk chocolate.