Line a tray or plate that fits in your fridge. Crush the biscuits by hand into uneven small chunks; do not make fine crumbs.
Toast the hazelnuts in a dry pan for 3–4 minutes, cool them, then chop roughly.
In a large bowl, beat the softened butter and sugar until pale and creamy.
Add the eggs one at a time, mixing well after each addition.
Sift in the cocoa powder and add the melted dark chocolate, vanilla, salt, and rum or milk. Stir until smooth and glossy.
Fold in the biscuit pieces and chopped nuts. Mix gently until every piece is coated and the mixture looks thick and moldable.
Place the mixture onto a sheet of baking paper or plastic wrap and shape it into a compact log about the size of a salami.
Wrap tightly, twist the ends, and roll it on the counter to refine the shape.
Chill for at least 4 hours, preferably overnight, until firm.
Unwrap, dust generously with powdered sugar, and rub lightly with your hands so it looks like a cured salami coating.
Slice with a sharp knife and serve slightly cool, not ice-cold, for the best texture.