Beat egg whites until soft peaks are formed. Gradually add the sugar, beating well after each addition. Continue beating until egg whites are very stiff. Fold in cashews and vanilla. Spread thinly on inverted baking sheets topped with silicone baking sheets. Bake at 350 degrees F for 25 minutes or until golden brown. Remove wafers while hot. You will need to carefully peel them off. Set aside to cool.
Boil sugar and water until it spins a thread (dip spoon in and lift to see if thin threads appear after drops). Meanwhile beat yolks with an electric whisk until thick. Pour syrup in thin streams while beating. Beat until very thick, creamy and cool (about 10 minutes). Switch from whisk to paddle or beater attachment. While beating, gradually add in butter. Add liqueur. Blend on medium high until fluffy and stiff (another 10 minutes).
Layer wafers and cream. Cut the cake in half and place one half on top of the other. Sprinkle extra ground cashew on top. Wrap in cling film and put in the freezer (for at least two hours). Take out and put in the refrigerator at least 1 hour before serving.