Heat oven to 180C/160C fan/gas 4 and line 2 baking sheets with baking parchment. Beat the butter and sugars in a bowl until light and fluffy, then beat in the eggs one at a time.
Add the flour, malted milk drink powder, baking powder and a good pinch of salt, then stir with a wooden spoon until the mixture forms a dough. Tip out onto the work surface and gently knead in the chocolate. Divide the mixture into 20 pieces, roughly the size of a whole walnut. Roll each piece into a ball and place on the baking trays, leaving plenty of room for the cookies to spread. Flatten each ball slightly with your hand.
Bake for 10-12 mins until starting to turn golden brown on the edges. Remove from the oven, leave on the baking tray for 5 mins to cool slightly then transfer to a wire rack to cool completely. Will keep in a cake tin or cookie jar for up to 5 days.