Grease and line a 3cm-deep, 15.5cm x 25cm (base) slab pan with 3cm overhang.
Place condensed milk and butter in a small saucepan over medium heat. Cook, stirring for 4 to 5 minutes or until butter melts.
Place biscuits in a food processor and process to fine crumbs. Combine crumbs, coconut, lemon rind and lemon juice in a bowl. Add hot butter mixture. Stir until well combined.
Press biscuit mixture into prepared pan. Refrigerate for 1 1/2 hours or until firm.
To make lemon icing: Place icing sugar mixture, butter and lemon juice in a bowl. Beat with a wooden spoon until smooth. Spread icing over slice. Top with coconut. Refrigerate for a further 30 minutes or until icing has set. Cut into pieces. Serve.