First up, assemble all of your ingredients. Make sure that you take the mascarpone cheese out of the refrigerator and include it in your assembled ingredients, as this gives it time to come closer to room temperature.
Next line a 9 inch square dish with a piece of parchment paper by cutting the parchment paper to fit.
In a large metal or glass bowl combine the egg yolks and sugar. Whisk together until mixture appears creamy and mostly smooth.
Add ¾ cup sweet sherry to the bowl and whisk momentarily to combine.
Now place a saucepan of water onto the stove and bring to a simmer. Reduce the heat to low. Place the mixing bowl on top of the saucepan, taking care not to let the base of the bowl touch the surface of the simmering water. If this does happen, pour or scoop out some of the hot water. This is what is termed a double boiler.
Keeping the water on a gentle simmer, continuously stir the egg yolk, sugar and sherry mix. As you stir the mixture over the double boiler, you will begin to notice it thickening and increasing in volume. After approximately 10 minutes of heating and stirring, the sabayon should be thick enough in consistency that a portion picked up on the whisk and dropped back in retains its form for a few moments. If you find that sabayon is not thickening, turn up the heat on the pot a little (the water should be simmering, but not boiling).
Once done, remove the sabayon from heat and allow it to cool for a few minutes. Then add in the mascarpone, whisking it into the sabayon until it is light and fluffy.
Now pour the cream into a separate bowl and whip until it forms stiff peaks.
Add the whipped cream to the sabayon mix, gently folding it in to combine.
In a small baking pan, bowl or other suitable container, combine the cold espresso and the leftover ½ cup sherry.
One by one, dip half of the lady finger biscuits into the coffee and sherry. Do this quickly, and definitely do not let the biscuits sit in the liquid as then they will become soggy and fall apart. The quicker they go in and out of the liquid the better.
As the lady finger biscuits come out of the coffee and sherry, place them in rows in the bottom of your lined 9 inch square dish.
Once the base of the dish is lined with finger biscuits, it is time to do a layer of the sabayon. Spoon in a generous amount of the sabayon over the finger biscuits and smooth with the back of a spoon.
Now place a thick layer of mulberries onto the layer of sabayon cream, and press them down a little. You can use up most of the mulberries, reserving a generous handful for decoration.
Spoon a second layer of sabayon over the mulberries, and smooth.
Repeat the dipping process with the other half of the lady finger biscuits, and place them onto the sabayon, pressing them down slightly to level them out.
Finally add a last scoop of sabayon on top of the lady finger biscuits, and with a cake knife or other suitable knife, spread evenly to create a smooth top. Place the completed tiramisu in the refrigertor and leave to set for 12 -24 hours.
Once set, remove from the refrigerator, dust generously with cocoa and top with fresh mulberries. Serve immediately.