Beat the egg yolks with the sugar until white and light. Add potato starch and limoncello and whisk with an electric mixer for about ten minutes. Fold in the whipped egg whites.

Preheat oven to 180°C, then butter and dust with potato starch a 26 cm round baking tin.

Scrape the batter into the tin and bake for about 30 minutes, until golden brown on the surface and dry inside. Test with a toothpick to be sure it is ready.

Let it cool and in the meantime make the Italian custard. Heat the milk over medium heat with the the vanilla bean until simmering. Remove from the heat.

Meanwhile, whisk the eggs with sugar and cornstarch in a saucepan until smooth and creamy.

Pour the hot milk in a thin stream into the beaten eggs and stir with a whisk to avoid eggs from curdling. Put the saucepan back on low heat and cook stirring constantly with a whisk for about 5-10 minutes, until thick and barely simmering. Remove from the heat and let it cool down.

When both the cake and the Italian custard are cool, slice the cake horizontally, brush with limoncello mixed with the same amount of water and spread it with custard.

Dust with icing sugar and decorate the surface with lemon verbena flowers. Keep the cake in the fridge for a few hours before serving, then enjoy it and happy birthday!