For the chocolate cake: Preheat the oven to 180 C / 160 C and lightly grease 3 x 7 inch round cake pans and line the bases with parchment paper.
Place the butter and coffee together in a large bowl and set over a pan of simmering water and heat, stirring occasionally, until the butter is fully melted. Remove the bowl from the heat and add the cocoa powder and chocolate and mix together until the chocolate is melted. Add the light brown sugar and vanilla and mix together until combined. Add the eggs and egg yolks and whisk into the batter until fully combined. Finally mix together the flour, baking powder and salt and sieve over the chocolate mixture, mixing together until just combined.
Divide the batter between the prepared pans, preferably weighing to make super equal layers, and bake in the preheated oven for about 30 - 35 minutes or until a skewer inserted into the cakes comes out clean. Allow the cakes to cool in the pans for 10-15 minutes before inverting onto wire racks to cool completely.
For the caramel Marmite buttercream: Melt the butter in a large pan over a low heat. Now add the brown sugar, milk and Marmite and stir until it starts to bubble. Let the mixture boil for 1 minute and then place into a large bowl. Taste the mixture and add more Marmite if necessary.
Now add half of the icing sugar using a whisk and leave to cool thoroughly mixing occasionally throughout the cooling. When fully cool mix through the remaining icing sugar if it is still runny add more until you have a spreadable consistency.
To assemble the cake, spoon about ¼ of the buttercream over one of the cakes and sandwich with the other cake then repeat with the third cake. Cover the whole of the cake generously with the buttercream using a palette knife and top with flaked chocolate. Enjoy!