Line a loaf pan with parchment paper, making sure there's some overhang for easy removal.
Crush the cookies or biscuits into small pieces and place them in a large mixing bowl
In a saucepan, melt the butter over low heat. Then, whisk in the cocoa powder until you have a smooth chocolate mixture.
Remove the saucepan from the heat and gradually mix in the sweetened condensed milk until well combined
Pour the chocolate and condensed milk mixture over the broken cookies in the bowl. Stir everything together until the cookies are thoroughly coated.
Transfer the mixture to your lined loaf pan. Press it down firmly to ensure it's compact.
Refrigerate for at least 4 hours, or until the cake is set and firm.
Use the parchment paper to lift the set cake out of the pan, slice it into bars or squares, and serve chilled.