Heat oven to 180°C.

Beat the sugar with the eggs until are light and foamy.

Sift flour, potato starch, baking powder and salt and then fold them with the grated zest of the lemon into the batter.

Add gradually the milk, whisking it thoroughly.

Butter and flour a 28 cm wide bundt cake mould and scrape in 3/4 of the batter.

Add 2 tablespoons of cocoa into the remaining batter, mix it well and then scrape it into the mold, drawing some doodle on the surface.

Bake the cake for 45 minutes, do the test with a toothpick to see if it is dry inside.

Let cool and then remove it from the mould.