Use a 20 cm square tin. The same mixture in a smaller tin is nearly twice as deep and takes far longer to set through. Line it with parchment overhanging two sides so you can lift the slab out to cut it.
Break the biscuits, do not blitz them. Leave plenty of big uneven pieces among the crumbs, or it sets dense and sandy instead of crunchy.
Real butter only. A spread or a blend carries water and the slab will not set firm.
Two hours is the minimum, overnight is better. A fully cold slab is what gives clean-edged squares.
Let it stand ten minutes before cutting, and use a knife warmed in hot water and dried. Cutting it fridge-cold is why the top layer cracks.