Preheat the oven to 180°C / 350°F / Gas mark 4.
Butter and line 2 x 20cm sandwich tins. Melt the butter and allow to cool slightly.
Put the eggs, sugar and vanilla into a bowl and whisk until the mixture is pale, thick and has trebled in volume about 8 minutes.
In a large bowl, sift the flour, cocoa, baking powder and a pinch of salt over the egg mixture, then gently bring together.
Pour the cooled butter around the edge of the bowl and mix in until fully incorporated.
Divide the batter between the tins and bake for 10-12 minutes or until a skewer inserted into the middle of the cakes comes out clean.
Allow to cool in the tins for a few minutes, then transfer to a wire rack to cool completely.
Make the coffee and dissolve the sugar in the coffee and set aside.
Put the egg yolks, sugar and marsala into a heatproof bowl, then set over a large pan of simmering water.
Whisk for 5 minutes until the mixture is thick and has trebled in volume – it should be quite thick.
Remove from the heat and take the bowl off the pan.
Keep whisking until the mixture has cooled.
Melt the chopped white chocolate in the microwave on a low heat and allow to cool slightly.
Meanwhile, whip the mascarpone until smooth, then fold into the egg mixture with the cooled white chocolate.
Place one layer in the bottom of the tin.
Pour ½ the coffee syrup over and spread with 6 tablespoons of the mascarpone mixture.
Scatter with 2 tablespoons of grated dark chocolate, then top with the second layer of cake.
Covering it with a generous dusting of cocoa.
Cover with cling film and chill for at least 2 hours along with the remaining mascarpone mixture on the side.
Remove the tin and put the cake on a serving plate, then cover with the remaining mascarpone mixture.
Mix the remaining dark chocolate with the grated white chocolate and press carefully onto the sides of the cake.
Dust the top with cocoa again and serve.