Preheat the oven to 325 degrees F. Prepare four 9-inch pans by greasing them and adding parchment paper to them. Set aside.
In a bowl, whip the egg whites until they become frothy. Add in the cream of tartar and continue to beat. Gradually (1 to 2 tablespoons at a time) add in the sugar and continue to beat until all of the sugar is incorporated and the egg whites are shiny and form stiff peaks.
Fold in the 1 cup of finely ground cashews and divide the meringue into the 4 pans equally. Bake in the oven for 30 minutes or until golden brown (mine were done after about 23 minutes).
While the meringues are still warm, remove them from the pans and parchment paper because the paper can be more difficult to remove once cooled.
When the meringues are fully cooled, line them up on top of one another to trim the edges evenly. Set aside.
Meanwhile, prepare the French buttercream. Beat the egg yolks until they thicken and double in volume.
Heat the sugar and water until the mixture reaches 235 degrees F (or thread stage). Keeping the mixer on high, slowly pour in the sugar mixture to the egg yolks.
Be sure to continuously beat so that the egg yolks do not cook and be careful not to burn yourself as well. I had to make Dave come and help me during this stage (he held the mixer while I poured the sugar in).
Once the sugar mixture is completely incorporated, continue beating on high speed for about 15 minutes, or until the mixture has reached room temperature.
Then, keeping the mixer on high, beat in the butter one tablespoon at a time until all of the butter in mixed in. Add in the flavoring of your choice and let set in the refrigerator for at least 1 hour.
To assemble, place a meringue layer down on a plate or cake board and spread a thin layer of buttercream on top. Add a second meringue layer on top of that and repeat until all of the layers are stacked and lightly frosted.
Finish by frosting the sides and top of the cake and by adding the rest of the chopped cashews around the sides for decoration.
Slice, serve, and enjoy!