Preheat oven to 180 degrees C. Line 3 baking trays with baking paper. Trace 23cm circles on 2 sheets of baking paper using a cake tin as a guide.
In a medium bowl, whip egg yolks and 1/4 cup of sugar with an electric mixer until thick and pale. In a separate bowl, whip egg whites to soft peaks using clean beaters. Gradually add the remaining 1/2 cup sugar while whipping whites to medium stiff peaks. Fold the egg yolk mixture into the egg whites, and gently fold in the flour and 1 teaspoon vanilla. The batter should be thick and pale yellow.
Spread or pipe batter to completely fill inside the lines of the circles. Batter should be about 1cm thick.
Load the remaining batter into a pastry bag fitted with a 5mm tip. Draw parallel lines 7cm apart on remaining sheet of baking paper. Pipe the batter back and forth between the lines in a compressed S motion, until you run out of batter. This piece will wrap around the inside of the cake tin. (It helps to have it in one piece, but you can pipe individual fingers using the guidelines drawn on the paper, if you prefer.) There may be batter left over.
Bake for 10 to 15 minutes, until firm but not browned, remove from the oven and dust generously with icing sugar. Set aside to cool.
To make the syrup, stir together 1 cup sugar, boiling water, coffee and 1/4 cup rum until sugar is dissolved. Set aside.
To make the filling, combine mascarpone, 2 cups icing sugar, remaining rum and vanilla in a large bowl. Whisk together until completely smooth. Gradually add the cream and whisk with an electric mixer until soft peaks form. Stop whisking when the mixture shows the first sign of graininess.
To assemble, line the sides of a 23cm springform tin with baking or waxed paper. Place one of the sponge finger rounds in the bottom, and brush it generously, but do not soak completely, with syrup. Place the 7cm sponge finger strips around the inside of the tin, covering the sides completely. Brush generously with syrup.
Spread half the filling mixture over the first sponge finger round. Place the remaining sponge finger round on top of the filling and soak it with syrup until it cannot take any more. Add the remaining filling and smooth the top. Sprinkle with grated chocolate and refrigerate for at least 4 hours.
To serve, remove the sides of the tin and carefully remove the baking or waxed paper from the outside of the cake. Dust with icing sugar just before serving.
Only just finished making this and it's already yummy... can't wait to try it tonight (6-7hours to settle)! I made it in rectangular pyrex dishes so I could put the lid on and take it to a friends for dinner - looks good layered through the glass. This recipe made lots of syrup - have left over and also left over biscuits/sponge (but they'll get eaten - nice and light as a biscuit).
I'm getting a big smile from my fiancee - i made a little sample one to keep him happy until after dinner! "Oh yum, can't wait to try it after it's been in the fridge!"
Remember to keep the chocolate cold to be able to grate it (I left it sitting by the oven and so I had to fridge it for a while, oops!) Also remember to whip the egg whites firm before adding the sugar (just sharing my silly mistakes in the hope it stops someone else making them). - 24 Jan 2010