Cream the softened unsalted butter in a mixing bowl with an electric mixer until light and fluffy, about 2 minutes.

Mix in the sweetened condensed milk on low speed until fully incorporated.

Sift together all-purpose flour, cocoa powder, salt, and vanilla extract in a separate bowl.

Gradually add the sifted dry ingredients to the wet mixture and mix on low speed until just combined.

Fold the mixture with a spatula until no streaks of flour remain.

Refrigerate the dough for 30 minutes or place it in the freezer for 15 minutes.

Roll the dough between parchment paper to 1 cm thick and cut out shapes.

Preheat the oven to 320°F (160°C) and arrange cookies 1 inch apart on a parchment-lined baking sheet.

Bake the cookies for 5 to 8 minutes until slightly firm.

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