Separate 6 eggs into egg whites and egg yolks. Let the 6 egg whites and 6 egg yolks come to room temperature.

Place 2 oven racks on the lower and upper third positions in the oven. Preheat the oven to 350 degrees F. Spread 3 cups of cashews in a large 13" x 18" sheet pan, and toast cashews in the upper third rack of the oven for 10 minutes, until they are golden brown. With the cashews still spread out on the sheet pan, set them aside and let them cool completely to room temperature (piling them on top of each other, like in a bowl for example, while still hot will make them soggy as they cool, which will affect the texture of the cashew powder).

Turn down the oven to 250 F (120 C). Line two large sheet pans (13x18 each pan) with parchment paper. I'm using a glass and a sharpie to make 3" diameter circles on the parchment paper so that there are 12 circles per pan, about 1/2 an inch apart. In each pan, I'm making 3 rows of circles, with 4 circles in each row. Flip the parchment paper over so the side with the sharpie marks faces down, and away from the cookies.

In a food processor, add in 1 cup of toasted cashews (half of the toasted cashews), 1/4 cup powdered sugar, and 2 tbsp of cornstarch. Pulse till they are mixed and the cashews are very fine - looks like a powder.

In the bowl of your stand mixer, add in the 6 room temperature egg whites and 1/2 tsp of cream of tartar. Beat on medium-high speed till the egg whites become frothy. Then very slowly add in 1/3 cup of granulated sugar. When all the sugar has been added, increase the speed to high. Continue beating till you see very stiff peaks.

Carefully fold in the cashew powder mixture into the meringue, one third at a time, as well as 1/2 tsp of vanilla extract, just until it is evenly mixed.

Transfer this mixture into a pastry bag fitted with no metal tip. At a 90-degree angle, use the prepared circles on the parchment paper as a guide to pipe the batter into circular wafer that are about 3 inches in diameter, 1/2 an inch high, with about half an inch between each wafer. I'm piping an outline of the circle first, then piping a dab in the middle, so that I can spread the mixture into a thin layer later.

Pipe 4 cookies per row, in the 3 rows, so that you have 12 wafers per baking sheet, for a total of 24 wafers.

If there's leftover batter after making 24 wafers, distribute the leftover batter equally over the cookies that have slightly less batter in them, by adding to the dab of meringue in the middle. Once all the batter has been piped, I'm using the back of a small spoon to spread the meringue from the sides and the middle to fill the circle, and smoothen and flatten the tops of the wafers as much as possible. Do this with all 24 circles.

Bake it in the preheated oven at 250 F (120 C) for 1 hour. Then turn off the oven heat but do not open the oven door. Leave the wafers in the oven for a minimum of 3 hours, to overnight, for the wafers to continue drying out.

The low temperature (250 F) and long baking time (1 hour + residual oven heat for 3 hours) is needed to remove all the moisture from the egg whites and create a hollow and dry wafer, without burning the outer layer of the meringue.

When the wafers are completely dry, they should come off the parchment paper quite easily. You can leave them at room temperature uncovered if you're not assembling the Silvanas cookie sandwiches immediately. This will allow them to dry out more (which is preferred).

French Buttercream:

Take 3 sticks of butter (1 1/2 cups) and slice them into 1-inch cubes. Let them come to room temperature.

Place the 6 egg yolks into the bowl of your stand mixer. With the whisk attachment, whip on medium-high speed for about 8 minutes, until thick and lemon-yellow in color.

In a small heavy bottomed saucepan combine 2/3 cup of granulated sugar and 6 tbsp of water. Set over medium heat, till sugar dissolves. Once sugar has dissolved, clip a candy thermometer onto the side of the saucepan. DO NOT STIR from now on. Turn the heat to highest possible, and heat mixture to soft ball stage, till thermometer reads 235 F (113 C). Immediately remove from the heat.

With the mixer running on medium speed, carefully drizzle the hot sugar syrup into the egg yolk mixture in a slow steady stream. This syrup needs to be hot enough so it's at the soft ball stage, and to also "cook" and pasteurize the egg yolks to make them safe to eat. Once all the sugar syrup has been added, stop the mixer to scrape down the sides of the bowl for any unmixed sugar droplets that may have splattered to the side, then turn the mixer back on to medium-high speed and continue to whipping until the mixture has cooled to room temperature, which may take about 15 to 20 minutes.

When the yolk-sugar mixture has cooled to room temperature, with the mixer running on medium speed, add the 1 1/2 cups of softened room temperature butter one cube at a time, allowing each piece of butter to be incorporated before adding the next. Once all the butter has been added, mix in 2 tsp vanilla, 1/4 tsp of salt, and an optional 2 tbsp of dark rum. Continue to beat until buttercream is silky and smooth - another 2 minutes.

NOTE: The butter will melt if the egg-sugar mixture is too hot. If you notice the butter separating, cool down the sides of the mixing bowl by placing it in an ice-bath. Alternatively, you can put the bowl in the fridge for a few minutes to cool the contents down before adding in more butter. Eventually, the buttercream should come together.

Assembling the Cookie Sandwiches:

In a food processor, add in the remaining 2 cups of toasted cashews (should be at room temperature) and 1/2 cup of powdered sugar, and pulse them till they are very fine crumbs, similar to the cashew powder we made in the beginning for the meringue. Transfer this cashew powder onto a shallow tray. Line a sheet pan, I'm using a 10.5" by 15.5" jelly roll pan, with parchment paper for placing the finished sandwiches.

NOTE: you can also use graham cracker crumbs for the outer coating. But using cashew crumbs stays closer to the "Sans Rival" flavor, and keeps this dish gluten free.

Take one of the cashew wafers, spread buttercream onto the uneven side (the side that was facing up during baking) so that it forms a thin layer of buttercream, about 1/4-inch layer of buttercream. Take another cashew wafer to sandwich the buttercream, pressing the uneven side against the buttercream. This way the flatter sides of both cookies are facing outward.

Spread more buttercream on the rim of the sandwich so the rims are even and flush with the wafers, and spread a very thin layer of buttercream on the flat faces of the sandwich. I'm putting just enough buttercream on the flat surfaces so that the cashew powder can stick to it. Don't worry about your fingers pressing into the buttercream, as any finger marks it will be covered by cashew crumbs eventually.

Place the buttercream-covered sandwich in the cashew crumbs, and cover all sides with the cashew crumbs. I start with covering the two flat sides with cashew powder, then rolling the edge of the wheel in the cashew powder to cover as much as possible, then doing the two flat sides again to cover up any finger marks. Then carefully place the finished Silvanas cookie sandwich on the prepared parchment paper-lined sheet pan.

Repeat this with the remaining meringue wafers, to make a total of 12 Silvanas cookie sandwiches, placing them on the parchment paper-lined sheet pan. I find it helpful to have a cloth handy nearby, and wiping any excess buttercream and cashew crumbs off before starting each cookie. Otherwise your fingers will get messier and messier with each cookie, making it more difficult to assemble the pieces.

Let the sandwiches rest in the freezer for a minimum of 1 hour, or in the fridge for a minimum of 3 hours to set. Once they've set, you can serve them immediately or store them in an airtight container. In the freezer, they will keep for up to 1 month. In the fridge, they will keep for up to 1 week.

They can be eaten straight from the fridge or the freezer, depending on your preferred texture. Enjoy!