Make the meringue shell. Start by preheating the oven to 350 °F.
Line a baking pan with parchment paper.
Combine the ground cashew nuts, chopped cashew nuts and all-purpose flour in a bowl. Mix and set aside.
Combine egg whites and cream of tartar in a mixing bowl. Begin whisking egg whites on low speed until it begins to foam. Increase speed to medium. Gradually add 1 cup sugar. Continue to whisk until shiny and medium peak forms.
Fold the flour mixture into the egg white mixture until all the ingredients are well-blended.
Transfer the mixture into a piping bag. Cut 2 centimeters from the tip. Pipe a circle shape and equally distribute the mixture on the pan line with parchment paper. Smooth the top with a spatula or scraper.
Bake meringue for 20 to 25 minutes. When the meringue shells are baked, immediately flip on another parchment paper and peel off the parchment paper from the bottom and allow to cool and set aside. Do this step 4 times to make 4 layers.
Make the Icing by combining water and ¾ cup sugar in a heavy bottom saucepan. Heat saucepan using low heat while stirring until sugar dissolves. Increase heat to medium-high and bring to a boil. Allow the syrup to cook until the temperature reaches 240°F. Immediately remove the saucepan from heat. (Note: a cooking thermometer is strongly recommended)
In a mixing bowl, combine egg yolks and vanilla extract. Whisk for 5 minutes using an electric mixer at a high-speed setting. Check the temperature of the syrup (this is the one made from the previous step). Once the temperature drops to 200°F, slowly drizzle the syrup into the egg yolk mixture. Continue mixing until the bottom of the bowl is cool to the touch and the yolk mixture has cooled to room temperature.
Add in butter, one cube at a time, allowing each piece to incorporate before adding the next. Continue mixing until buttercream is smooth and creamy. (About 5-6 minutes.)
Assemble the cake by filling the meringue shells with buttercream, put on top of each other. Spread more icing on top and sides of the meringue shells until the cake is fully covered with icing.
Sprinkle the cake with chopped cashew nuts or decorate as desired.
Serve and enjoy!