Preheat the oven to 180 degree Celcius. Grease a 12x10x1 inch pan and line with baking parchment paper.

Place the eggs and sugar a mixing bowl and, using an electric mixer, whisk together until the mixture is very pale and thick (this is a must, otherwise when the butter mix in, it can deflate the batter so that the cake will be dense). The mixture should leave a light trail on the surface when the whisk is lifted.

Sift over the flour and fold in gently using a spatula, taking care not to over mix. Take about a fourth of the batter and fold it into the melted and vanilla extract with a spatula. When completely combined, fold the mixture into the remaining batter.

Pour the batter into the prepared cake and bake for 17 - 20 minutes, or until risen, golden-brown and springy to the touch. Cool in the tin for five minutes then turn out onto a wire rack and leave to cool completely.

For the syrup, dissolve the coffee and sugar in the hot water and add the kahlua. Set aside to cool.

In a large bowl, beat mascarpone cheese, sugar, coffee syrup and the vanilla with electric mixer on medium speed until smooth.

In small bowl, beat heavy cream with electric mixer on high speed until stiff peaks form. Gently fold whipped cream into mascarpone mixture until combined.

To assemble :
When the sponge is cold, carefully cut the cake into 3 pieces, each cake would be 3×10×1 inch.

Place one layer of sponge, moisten generously with coffee syrup. Then spread the mascarpone filling over the soaked sponge. Scatter over the grated chocolate.
Place the second layer on top and repeat the process.

Place the third sponge on top and spoon over the remaining coffee mixture. Using a palette knife spread a very thin layer of the remaining filling all over the cake – this is called a ‘crumb coat’ and will seal in any loose crumbs of sponge.
Wipe the palette knife and spread the rest of the filling in a thicker layer all over the cake. Sprinkle with the grated chocolate.
Chill for at least three hours in the fridge before serving.

Note :
If you don't have kahlua on hand, add 1/8 tsp coffee instant and 30 ml hot water to replace kahlua.