Preheat your oven to 200°F (93°C) and line two baking sheets with parchment paper.

Whip the egg whites until soft peaks form, then gradually add granulated sugar while increasing mixer speed until stiff peaks form.

Fold in powdered sugar, salt, cake flour, and finely ground pistachios until just combined.

Spoon the meringue mixture onto prepared parchment in two 7-inch circles; bake for 1.5 hours until dry and crisp.

Turn off the oven and let meringues cool completely inside to prevent cracking.

In a saucepan over medium heat, combine egg yolks, granulated sugar, and water; stir until thickened.

Allow the mixture to cool, then whip unsalted butter until light and fluffy; gradually mix in cooled yolk mixture.

Add pistachio paste and salt, beating until smooth and creamy.

Once completely cool, remove meringue layers from parchment, layer with buttercream, and chill for 30 minutes.