Line a 9 x 5-inch loaf pan with parchment paper, leaving paper overhang on the long sides. Break the biscuits into uneven bite-size pieces in a large bowl, keeping some larger chunks for a satisfying crunch.

Melt the butter in a medium saucepan over low heat. Whisk in the sugar, cocoa powder, milk, and salt until smooth, then cook gently for 3 to 4 minutes without allowing the mixture to boil hard.

Remove the saucepan from the heat and stir in the vanilla extract. Let the chocolate mixture cool for 2 to 3 minutes so it is warm and pourable rather than extremely hot.

Pour the chocolate mixture over the broken biscuits and fold carefully with a spatula until every piece is coated. Work gently so the biscuits stay chunky instead of turning into fine crumbs.

Transfer the mixture to the prepared loaf pan and press it down firmly, especially into the corners. Smooth the top with the back of a spoon, cover the pan, and refrigerate for at least 4 hours.

Lift the chilled cake from the pan using the parchment handles and place it on a cutting board. Drizzle with melted dark chocolate or dust with cocoa, then slice with a sharp knife and serve cold.