Preheat the oven at 180°C.
In a bowl, beat the butter with 2/3 sugar till the mixture is whitened, airy and fluffy.
Separate egg whites and egg yolks. Set the whites aside in the fridge. Mix the yolks into the butter base one by one.
Mix flour with cornstarch and add to the base.
Whip the egg whites with left sugar and carefully fold into the mixture. Don't put everything at once.
In a small bowl, pour 3 tbsp of dough and mix with grinded poppy seeds.
In a round cake pan with baking paper, pour mixture without poppy seeds, make it nicely flat and add chinks of poppy seeded dough on top.
Bake the base for around 30 minutes or until it's golden-brown and the toothpick comes out clean. Then let it cool down.