Line a 20-23 cm (8-9in) Springform tin with bakingpaper. The smaller tin will make a deeper cake with a thicker base and vice versa for the larger tin.
Blitz the biscuits in a food processor until they turn to crumbs. Melt the butter, add the salt, then blend again until the crumbs start to clump together. Tip the crumb mixture into the tin, then flatten into a