First make the sponge. Preheat the oven to 350 degrees F (180 degrees C /gas 4).
Whisk the sugar and eggs until they are at ribbon stage. Fold in the melted butter, then fold in the sifted flour and cocoa. Pour the mixture into a lined Swiss roll tin and bake in the preheated oven for 10 minutes. Remove the sponge from the oven when it is done and leave to cool.
To make the filling, put the mascarpone, sugar, egg yolks, and Vin Santo into a bowl and mix until smooth.
To assemble the tiramisu, break up the sponge and press it into the bottom of a shallow dish. Drizzle over the coffee, Tia Maria, and white chocolate. Spoon over the mascarpone filling, then dust liberally with cocoa. Using a large knife, scrape the chocolate towards you to make shavings and arrange these delicately over the top.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.