Preheat the oven to 160°C / 320°F. Line three 20 cm / 8-inch round cake pans with baking paper.

Process 350 g pistachios in a food processor until finely ground, stopping before they become pistachio butter.

Combine the ground pistachios with 20 g all-purpose flour and 1/2 tsp baking powder in a mixing bowl.

Whip 400 ml room-temperature egg whites in the stand mixer until foamy. Gradually add 230 g granulated sugar while mixing.

Continue whipping the egg-white mixture to stiff, glossy peaks. Beat in 1 tsp vanilla extract.

Fold the pistachio-flour mixture gently into the meringue until no dry streaks remain. Divide the batter evenly among the prepared pans.

Bake the pistachio meringue layers for about 25 minutes, until set and lightly golden. Cool the layers completely on wire racks.

Whisk 5 egg yolks, 150 g granulated sugar and 50 g corn flour in a mixing bowl until smooth.

Heat 550 ml whole milk in a saucepan until steaming but not boiling. Whisk a small amount of hot milk into the yolk mixture.

Pour the tempered yolk mixture into the saucepan. Cook over medium heat while whisking until the pastry cream thickens and bubbles.

Remove the pastry cream from the heat and whisk in 50 g unsalted butter, 2 tsp vanilla extract and 1 Tbsp orange blossom water if using.

Press plastic wrap directly onto the pastry cream surface. Refrigerate it until fully chilled, at least 2 hours.

Whip 250 ml whipping cream to medium-stiff peaks. Fold the whipped cream into the chilled pastry cream until smooth and light.

Place one cooled pistachio meringue layer on a serving plate. Spread half of the pastry-cream filling evenly over the layer.

Add the second pistachio meringue layer and spread over the remaining pastry-cream filling. Top with the final pistachio meringue layer.

Whip the remaining 300 ml whipping cream with 80 g icing sugar and 1 tsp vanilla extract to stiff peaks. Frost the top and sides of the cake.

Sprinkle 1 to 2 Tbsp coarsely ground pistachios over the frosting. Refrigerate the assembled cake for at least 1 hour before slicing.