Line a large baking sheet with nonstick baking paper. In a large bowl, beat the butter and sugar together until light and fluffy. Mix in the vanilla extract, then gradually beat in the egg, beating well after each addition. Stir in the milk.
Sift the flour and baking powder into a separate large bowl, then fold into the butter and egg mixture. Stir in the chocolate chips.
Drop level tablespoonfuls of the cookie mixture on to the prepared baking sheet, leaving about 3.5 cm (1½ inches) between each cookie, then lightly press with a floured fork. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 15 minutes or until lightly golden. Transfer to a wire rack to cool.