Lightly butter 3 baking trays. Break up the chocolate and gently melt it along with the butter in a heatproof bowl set over a pan of barely simmering water or in a microwave, taking care not to burn the chocolate, stirring occasionally. Stir in the condensed milk, then remove from the heat and cool. Mix in the flour and the chocolate buttons and chill the mixture until firm enough to handle. Preheat the oven to 180C / fan 160C / gas 4. Place large teaspoonfuls of the mixture spaced well apart on the prepared baking trays. Bake in the preheated 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.