Preheat the oven to 170ºC/325ºF/gas 3. Line two baking sheets with greaseproof paper.
Beat the butter and sugar in a large bowl until pale and creamy. Crack in the egg, add the vanilla and mix well. Sift in and fold through the flour and salt. Roughly chop and stir in the chocolate.
Roll tablespoons of the dough into balls and place onto the lined trays. Cover and chill in the fridge for 15 minutes.
Flatten the balls slightly with your fingers, then place in the hot oven for 10 to 12 minutes, or until lightly golden (make sure you don’t overdo them – the chewier the better). Leave to cool completely, then serve with a glass of cold milk.