Lightly greased 3 baking trays. Break up the chocolate and gently melt it along with the butter in a bowl set over a pan of simmering water, stirring occasionally. Stir in the condensed milk then take it off the heat and cool.
Mix in the flour and the chocolate buttons and chill the mixture until firm enough to handle. Pre-heat the oven to 180 degrees Fan 160 degrees Gas 4.
Place large teaspoonfuls of the mixture spaced well apart on the prepared baking trays. Bake in the pre-heated oven for about 15 minutes. The cookies should still look soft and will glisten. Don’t overcook them as they soon become very hard. Carefully remove the cookies with a palette knife and cool on a wire rack.