Beat together the butter and sugar in a bowl until combined. Beat in the egg until smooth and creamy. Add the flour and chocolate to the bowl and mix to a paste.
Roll heaped teaspoonfuls of the mixture into balls and place, well spaced apart, on 2 greased baking sheets.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for about 15 minutes or until the cookies have spread and are pale golden. Leave to harden on the sheets for 2 minutes, then transfer to a wire rack to cool.