Using a hand or stand mixer, beat egg whites for a minute using medium speed. Sprinkle the cream of tartar and go on beating until soft peaks form. Gradually add the granulated sugar and start beating at medium-high speed until it forms stiff peaks.
Fold in the ground cashew in 2-3 parts until it is well distributed.
Transfer meringue into a piping bag. Pipe meringue onto a cookie sheet lined with parchment or wax paper making circle or oval shapes. Draw a pattern beforehand to make it easier to pipe and cookies even.
Bake meringue on the middle rack of a preheated oven at 150°C /300°F for 25-30 mins or until lightly browned.
Meanwhile, prepare the buttercream by beating the butter and the powdered sugar together for a minute at high speed.
Remove from oven and let them cool completely before detaching from the parchment or wax paper. You could easily remove them if they are cooked through.
To assemble, spread some buttercream on a meringue cookie and sandwich it with another cookie, both with the flat sides facing outside. Squeeze them lightly together then remove the excess buttercream from the sides using the dull side of a bread knife, add more buttercream if it is too less, the sides should be totally filled. Spread also the flat side with a thin layer of buttercream. Roll the meringue cookie sandwich in biscuit crumbs covering everything.
Place the Sylvanas in the fridge for at least some minutes before serving.