Preheat oven at 350 degrees F. Line a 12x18in cookie sheet (or 3 9in rounds) with a parchment paper.
Finely chop the cashew for the meringue.
Beat the egg whites and cream of tartar on medium speed until foamy.
Add the sugar. Beat until hard peaks.
Fold in the cashew.
Spread the meringue in pans as thinly and as evenly as possible. (thickness about 1cm) Bake for 25mins or until golden brown. Let cool.
Whip the butter until creamy.
Beat egg yolks in a separate bowl.
Boil sugar and water in a saucepan. Check the syrup consistency by pouring half a teaspoon of syrup into cold water. If the syrup forms a soft ball, the syrup is done. Be careful not to overcook it or it will crystalize.
In a steady stream, pour the syrup into the egg yolks while beating at high speed.
Fold in the whipped butter and cashew nuts.
If using 12x18in baking sheet, cut the meringue in 4 and remove from parchment paper. If using the round pans, simply remove parchment paper.
Set aside 1/3 cup of the filling. Alternately layer the meringue and the remaining filling starting with meringue. Use the 1/3 cup to coat the sides. Pour excess on top.
Cover the top and sides of the cake with the remaining nuts.
Freeze for about an hour and let chill for about 2 hours before serving.