Preheat oven to 190°C.

Whisk butter with sugar until white and soft. Add the eggs one at a time, whisk until perfectly incorporated.

Sift the polenta flour with the potato starch, the baking powder and a pinch of salt. Fold them into the batter along with the grated zest of a lemon.

Grease a 24 cm round baking pan, sprinkle with polenta flour and shake off the excess flour. Scrape the batter into the baking pan and bake for about 55 minutes, until golden brown.

Let cool completely before serving with a generous dusting of icing sugar.