Crack the eggs into a heat resistant bowl, then add the 200 g sugar, vanilla sugar or vanilla extract and dark chocolate. Stir well with a wooden spoon.
Put the bowl over a saucepan of boiling water, or use a double boiler. Stir the egg mixture continuously over the steam for 10 minutes, trying not to stop as it may scramble.
Set aside to cool to room temperature.
Meanwhile, beat the butter and 50 g caster sugar for 5 minutes until light and fluffy. Whisk in the cooled chocolate mixture until combined, then refrigerate until cooled completely.
Dip 6 of the biscuits into the espresso and arrange them in a 3 x 2 rectangle on a serving board or plate. Only dip the biscuits for a second as you don’t want them soggy.
Evenly spread some of the chocolate filling over the dipped biscuits. The filling should be a little less than 5 mm thick.
Dip 6 more biscuits into the espresso and lay them over the first layer, then keep repeating the process until all of the biscuits are used up. This will give you 7 layers of biscuits.
When the cake is assembled, spread the filling over the top and sides of the cake. Refrigerate for a few hours, or until the topping is firm again. Gently cover it with plastic wrap and refrigerate for a couple of days, or at least overnight, before slicing and serving.