Photograph: Dan Jones
Preheat the oven to 150°C, fan 130°C, gas 2. Using an electric hand whisk or mixer, cream the butter and sugar together until pale and fluffy.
Beat in the condensed milk, then add the flour, cocoa and a pinch of salt, and mix. Stir in the chocolate chunks.
Roll into 12 walnut-sized balls and space at least 5cm apart on 2 baking sheets, then flatten them slightly with the back of a spoon. Bake for about 25 minutes, until firm at the edges but still soft in the middle. Leave to cool for a few minutes before transferring to a wire rack.
Get ahead: make a few days in advance; store in an airtight box. Can be frozen.
Kitchen secret: You can vary this American-style cookie recipe in all sorts of ways. Try swapping the chocolate chunks for chopped nuts, glacé cherries, fudge chunks, dried fruit or crystallised ginger.