Start by preheating your oven to 180oC, then grease and line a 40cm x 30cm rimmed cookie tray with baking paper.
Place the egg whites into a large, clean bowl and the yolks into a separate medium bowl. Next, whisk the egg whites using an electric whisk until the peaks are nice and soft. Then gradually whisk in 50g of the sugar until the mixture turns stiff and glossy. Then, gradually whisk the remaining 50g of sugar into the yolks using the electric whisk until pale and thick.
Pour the yolk mixture and the vanilla into the whipped whites and combine with the electric whisk. Mix some of the beaten egg mixture into the melted Lurpak® Unsalted butter, then pour this mixture back into the large bowl. Sift the flour and cornflour into the large bowl, too. Fold everything together gently using a spatula until just combined. Pour onto the prepared cookie tray and spread out over the whole tray in an even layer. Bake for 15 minutes – the cake should be evenly golden and crisp at the edges
Dust a clean kitchen towel with icing sugar and flip out the hot cake on top. Next, peel off the baking paper and dust the whole cake with more icing sugar. Trim all the edges of the cake so it’s a ~28cm square – you can eat any of the extra cake trimmings! Starting at the short edge, tightly roll the hot cake up with the towel (so the towel is literally rolled in between the cake layers, not just around the outside). Then leave the cake to cool completely.
Make the filling:
Pour the double cream into a large bowl and whisk until thickened. Add the mascarpone and stir that in until smooth. Pour in the Marsala, icing sugar and vanilla. Stir vigorously until the mixture is smooth and keep whisking until it thickens up again. Chill until needed later.
Make the coffee soak:
In a medium bowl combine the boiling water, instant coffee and Marsala. Stir until the coffee has completely dissolved.
Next, unroll the cooled cake and brush all over with the coffee mixture using a pastry brush. Be sure to use all of the mixture. Spread the filling mixture over the surface of the cake stopping just short of the edges. Then roll the cake up again (starting at the short edge as you did before). Transfer to a wire rack and chill while you make the glaze.
For the glaze:
Place the Lurpak® Unsalted butter, cream and chocolate into a small saucepan over a low heat. Stir gently and constantly until the chocolate has melted. Then take off the heat and stir in the golden syrup and milk.
Remove the cake from the fridge, still on the rack, and set it over a baking tray. Pour the glaze all over the cake, coaxing it with a spoon if needed, so that it covers everything.
Leave to cool for a few minutes then use a metal spatula and your hands to transfer the cake to a serving platter. Decorate with shaved white chocolate and cocoa nibs. Chill for at least 30 minutes and up to two days before serving.