Cover a small (approx. 20x30cm) rectangle baking tray with parchment paper and preheat an oven to 180C.

Beat the butter with sugar until pale and fluffy.

Whisk the egg whites until stiff.

Mix the ground pistachios and flour and stir into the beaten butter.

Add half of the whisked egg whites to the pistachio mixture and stir to loosen it up, then add the rest of the egg whites and stir again, this time very gently to keep as much air as possible.

Pour the butter into the prepared tray and bake approx. 30 min. until a stick inserted in the middle of the cake comes out clean.

While the cake is in the oven prepare the top: beat the egg yolks with sugar until pale and fluffy (3-5 min.); stir in the potato starch.

When the cake is baked, take it out from the oven and immediately spread the egg yolk mixture on top and decorate with pistachios if you wish.

Put it back in the oven for 5-7 min (until the top is baked).