Brown the butter until deeply golden and nutty. Pour into a bowl and cool until barely warm.
Rub the lemon zest into the caster sugar with your fingertips.
Whisk the eggs and lemon sugar for 2–3 minutes until lighter and slightly aerated. Whisk in vanilla and gradually whisk in the brown butter.
Combine almond meal, polenta, cornflour, baking powder and salt.
Fold the dry ingredients into the egg mixture, alternating with the milk. Fold through the lemon juice last.
Pour into a greased and lined 20cm springform or cake tin.
Bake for 35–45 minutes, until golden and a skewer inserted into the cake itself comes out clean. Cool completely.