A rich and dreamy white and dark chocolate layered cake
First make your ganache:
Place the chocolate in a medium-sized, heatproof bowl.
Bring the cream just to a boil in a small saucepan set over low-medium heat.
Pour the cream over the chocolate and let stand, without stirring for 5 minutes or until the chocolate begins to melt.
Using a medium-size spatula, gently stir the chocolate and cream until completely combined; begin stirring near the centre and slowly work your way toward the edge of the bowl, pulling in as much as possible, until the mixture is smooth.
Add the liqueur (if using) and stir to combine.
Let stand until cool and thickened.
Next make your cakes:
Heat oven to 160° C (325° F). Butter and line with non-stick baking paper, 2 (8 inch) square baking tins; set aside.
Place the white chocolate in a heatproof bowl, set over a saucepan of just simmering water, and stir with a spatula until melted and smooth, set aside.
Repeat with the dark chocolate.
In the bowl of a stand mixer fitted with the paddle attachment at medium speed, mix together the butter, eggs, milk, caster sugar, baking powder and flour and beat until creamy.
Divide the batter into two separate bowls.
Add the coffee to the melted dark chocolate and stir to combine. Add the vanilla to the white chocolate and stir to combine.
Add the dark chocolate mixture to one of the bowls of batter and stir to combine. Pour the batter into one of the prepared baking tins.
Add the white chocolate mixture to the remaining bowl of batter and stir to combine. Pour the batter into the remaining baking tin.
Bake for 20 to 25 minutes or until a cake tester inserted in the centre of the cake comes out clean.
To assemble and decorate
Using a large bread knife, slice the cooled cakes in half horizontally.
Transfer one of the dark chocolate cake layers to a plate (cut side up). Using an offset spatula spread a layer of the dark chocolate ganache on top of the cake. Lay one of the white chocolate cake layers on top of the dark chocolate ganache (press down gently) and spread a layer of dark chocolate ganache. Repeat with another layer of dark chocolate cake and dark chocolate ganache.
Place the final layer of white chocolate cake, cut side down (press down gently). Frost the top and sides of the cake with the remaining dark chocolate ganache.
Spoon the melted white chocolate into a piping bag fitted with a fine nozzle (or use a sandwich bag and snip off the end instead). Pipe the white chocolate in a zigzag motion over the cake.
Decorate with whipping cream (optional).
Slice and serve.