Beat the softened unsalted butter in a mixing bowl using an electric mixer on medium speed until light and fluffy, about 2 minutes. Then add sweetened condensed milk and mix for another minute.

In a separate bowl, sift together the all-purpose flour, cocoa powder, and salt. Gradually fold the dry mixture into the butter and condensed milk mixture until just combined.

Wrap the dough in plastic wrap and refrigerate for 30 minutes or freeze for 15 minutes to make it easier to handle.

Dust your work surface with flour and roll the dough between two parchment paper sheets to about 1 cm thick. Cut into shapes and space about 1 inch apart on a baking sheet.

Preheat your oven to 320°F (160°C) and bake the cookies for 5-8 minutes until slightly firm. Remove from oven and allow to cool.

Let the cookies cool on the baking sheet for 2-3 minutes before transferring to a wire rack to cool completely.