Heat the oven to 200°C/180°C fan/400°F/gas mark 6. Grease a 20cm/8 inch square cake tin and line with baking parchment.
Place the cornmeal, flour, baking powder, bicarbonate of soda, salt and sugar in a large bowl and whisk well to combine.
Add the milk, vinegar and melted butter and whisk until no dry lumps remain.
Pour the batter into the prepared tin and spread level. Bake for 20-30 minutes until a skewer inserted into the centre comes out clean.
Leave to cool in the tin then slice into squares and serve. Store any leftovers in an airtight container for up to five days.
