Line and grease an 8″ / 20cm square baking pan. Crush the biscuits using a food processor and use a heavy object like a rolling pin. Place the crushed biscuits and coconut in a medium sized bowl.
Melt the butter and add to the biscuits and coconut along with the lemon zest and sweetened condensed milk. Mix the slice base together until well combined and pour out into the pan.