1 ½cupsunsalted butter at room temperature and cut into large cubes
Preheat the oven to 300 degree F. Grease and flour four 8-inch round baking pan. Line the bottom of each pan with parchment paper. Set aside.
To make the meringue shells
Mix together the ground cashew, chopped cashew and flour in a bowl. Set aside.
In a bowl of stand mixer, combine the egg whites and cream of tartar. Beat the mixture at medium speed while adding the sugar one tablespoon at a time. Increase the speed to medium high and continue beating until medium peaks are formed, about 4 minutes. Fold-in the flour mixture 1/3 at a time into the egg white mixture until well blended.
Equally distribute the batter among the prepared pans. Using a spatula, spread the batter until smooth. Place in the center of the oven and bake for 35 to 40 minutes or until the meringue is golden brown.
Remove from the oven. Immediately run a butter knife around the edges of the pan and invert pan into a cooling rack to release the meringue shell. Peel off the parchment paper from the meringue shell and set aside to cool.
To make the buttercream frosting
Place the egg yolks in the bowl of a stand mixer. Whisk at medium speed until pale in color, about 5 minutes. Set aside.
Combine the water and sugar in a small saucepan. Let the sugar mixture boil over medium high heat. Use a candy thermometer to monitor the temperature of the syrup. Without stirring, cook the sugar syrup until it reaches 240 degrees F. Remove from the heat and transfer the syrup into a glass measuring cup.
With the mixer running on medium speed in the yolk mixture, pour the sugar syrup in a slow steady stream. Once all the syrup has been added, turn the mixer to medium high and whisk for 8 to 10 minutes or until the yolk mixture has cooled down. Add the butter one cube at a time, whisking the mixture for about 20 seconds after each addition. Add the vanilla and continue whisking for another 5 minutes or until the mixture is silky and smooth.
To assemble the cake
Place one meringue shell on a cake turntable. Spread a layer of buttercream on top of the shell. Sprinkle top with the chopped roasted cashew. Repeat the process with the other two meringue shells, finishing off with the last meringue shell on top. Cover the top and sides of the cake with the buttercream frosting. Sprinkle top with the chopped cashews. Chill in the fridge for at least 2 hours or overnight to set.