Line a 9x5-inch loaf pan with parchment paper or cling film.
In a large mixing bowl, whisk melted butter, condensed milk, cocoa powder, and vanilla until smooth and glossy.
Add broken biscuits and optional nuts/chips. Gently fold until all pieces are well coated.
Transfer mixture to the prepared pan and press down firmly with the back of a spoon or spatula.
Cover and refrigerate for at least 4 hours or overnight until completely firm.
Unmold, slice into pieces, and serve chilled or at room temperature.