Using a stand mixer fitted with the whisk attachment (or a handheld electric mixer), whisk 10 egg whites on medium high speed until foamy.
Add ¾ teaspoon cream of tartar and continue whisking until soft peaks.
Gradually (about a teaspoon at a time) add the 1 cup sugar until stiff peaks form.
Remove the bowl from the mixer and gently fold your 1 cup toasted cashews.
Transfer your meringue to a piping bag and pipe 4 rectangles onto your prepared cookie sheet/s. Space out evenly, at least 2 inches apart, and smoothen the tops. See notes.
Bake for 30-40 minutes or until the top turns a light golden brown. Once that happens, turn the oven off but leave the meringue in the oven with the door closed for another 20 minutes.
Take the meringue out of the oven and leave on the cookie sheet until it's cool enough to handle (don't leave on the pan for too long or they will become too brittle and break).
Carefully peel the meringue off the parchment and trim the sides if needed so they're even. Cool completely before assembly.
For the French Buttercream:
While your cookies are baking, you can work on your buttercream. Start by boiling water in a large pot, then bringing it to a simmer.
In a large stainless steel bowl (I like using the bowl of my stand mixer), whisk 10 egg yolks, ⅔ cup + 2 tsp granulated sugar, 1 tbsp vanilla extract and a pinch salt until combined.
Place bowl over simmering water and constantly stir and scrape the sides so the egg yolks don’t scramble. You need this egg sugar mixture to reach 155F/68C to make sure the yolks are cooked through.
When it’s ready, transfer the bowl to your stand mixer fitted with the whisk attachment and whisk at high speed until it’s light and smooth (6-8 minutes). If you touch the bottom of your bowl it should not be hot anymore.
With the mixer still on, start adding your butter one piece at a time and continue whisking until your buttercream is thick and creamy.
If you’re not using the buttercream right away, transfer to an airtight container and refrigerate. You’ll need to re-whip before using.
For the Assembly:
Place one meringue layer on a cake stand or platter or turntable and spread about ¼ of your buttercream evenly on it.
Sprinkle with toasted cashews and continue to do the same thing for the rest of your layers. For your last one, make sure to put the smooth side up so your cake looks nice and even on top.
Then you can go decorate! Frost your entire cake with buttercream and slap on the rest of your cashews on the sides or tops or both.
Refrigerate for at least 2 hours to allow the cake to set. Leave on the counter at room temperature for about 10 minutes before serving. Enjoy!
Video
Notes
It’s helpful to create a guide or template to help you pipe your meringue later. Simply trace 4 rectangles that’s roughly 8x5 inches each on your parchment paper and flip it over, then lightly grease. If your rectangles don’t fit in one baking sheet, use 2 pans.
Instead of rectangles, you can also use round pans to make a round cake.
Note that there's enough meringue to make 4-6 layers; I just always make 4 fairly thick meringue pieces. You can make slightly thinner meringues to make a taller layer cake.