Preheat your oven to 210°F (100°C) while whipping the egg whites until foamy. Gradually add granulated sugar until stiff peaks form. Gently fold in ground pistachios, powdered sugar, and sifted cake flour until combined.

Line baking sheets with parchment and pipe meringue into three 7-inch circles, smoothing with a spatula. Bake for 1.5 to 2 hours until dry, then cool completely in the oven.

In a saucepan, combine water and granulated sugar over medium heat until dissolved. Beat egg yolks until pale, then drizzle in the syrup while whisking. Add butter and pistachio paste, beating until creamy.

Once meringue layers are cool, place one layer on a plate, spread buttercream on top, and repeat with all layers, using leftover buttercream for the top or sides.

Refrigerate the cake for at least 1 hour before serving. Allow to come to room temperature for best taste. Slice and enjoy!