First Combine the coffee and alcohol: Dissolve the coffee in the boiling water, then add 100ml cold water and the grappa, or sherry or Tequila. This is an important step, drink a tot of whatever you just put in! :)
Sponge layer: Arrange the sponge fingers in a 20cm square shallow dish (or tin), making a double layer. Spoon the coffee mixture evenly over the biscuits.
Topping: Beat the sugar and yoghurt into the mascarpone, and then spread this over the biscuits, pressing down well once they are all covered. Spread out the chocolate and press down. Chill for 10 minutes in the freezer, if possible and voila.
For the marscarpone: Place cream in a glass casserole or bowl, and place casserole into a larger flameproof pan. Add cold water to a larger pan. Place the pan over medium heat and bring the cream to a temperature of 180 degrees (75 to 80 Celsius). Use a candy thermometer), stirring every so often with a wooden spoon. As soon as the cream reaches the EXACT temperature, remove from the heat, add tartaric acid, and stir with a wooden spoon for 30 seconds. Remove glass casserole or bowl from the larger pan, and stir another 2 minutes. Line a fine-mesh basket or strainer with thick cheesecloth and pour in cream mixture. Allow to stand for 12 hours in a cool place or on the bottom shelf of the refrigerator. Prepare four 9inch squares of cheesecloth. Divide mascarpone in four. Place a quarter of it on each square of cheesecloth and fold like a package, without tying it. Place packages on a plate and refrigerate for another 12 hours before using.