In a large bowl whisk 500g mascarpone cheese and lemon zest. Then 100g chopped blueberries.
In a separate bowl, combine 4 egg yolks and sugar, continue mixing with a whisk or electric mixer until the sugar has dissolved and egg yolks are lighter in texture.
With a rubber spatula, gradually mix and fold to combine the egg mixture and the mascarpone cheese.
Prepare the meringue. Simply add the egg whites to one bowl and add the sugar gradually, whisk with a hand whisk or a mixer until it is light and fluffy. Pour into the mascarpone mixture then whisk until combined.
Add 4ml of Purple Yam flavoring to achieve a very nice purple and ube yam flavor to the mascarpone mixture.
For the assembly, dip the lady fingers top and bottom into espresso coffee. (You may also use normal or arabic coffee. Make sure the coffee is cold.) Arrange the lady fingers at the bottom of the cake pan.
After the bottom layer of dipped ladyfingers are arranged, pour the mascarpone mixture gradually (1/3 at a time) on top and smooth it out using an offset or rubber spatula. Repeat the process until you achieve the desired height of your tiramisu cake.
Top with marinated blue berries and chill overnight. (You may also add some lady fingers or chocolate shaves on the edges of the cake for decoration).