250g/9oz plain flour
½ tsp baking powder
½ tsp bicarbonate of soda
110²µ/3¾´Ç³ú light brown sugar
110²µ/3¾´Ç³ú caster sugar
½ tsp sea salt flakes, plus extra for sprinkling
½ tsp vanilla extract
250g/9oz chocolate (see tip)
110²µ/3¾´Ç³ú unsalted butter, diced and at room temperature
1 large free-range egg