Grease and line a 3cm-deep, 15cm x 25cm (base) slab pan or 20cm square pan
Place condensed milk and butter in a small saucepan over low-medium heat. Cook, stirring for 4 to 5 minutes or until butter melts. This mixture burns easily, keep stirring and do not leave unattended.
Combine crumbs, coconut and 2 teaspoons lemon rind in a bowl. Add hot butter mixture. Stir until well combined.
Press biscuit mixture into prepared pan. Refrigerate for 2 hours or until firm
Place icing sugar in a medium to large bowl, make a well in the centre, then slowly add lemon juice, stirring to make a slurry.Using this method reduces the risk of lumps and results in smoother icing
Add the melted butter to the slurry, mix well then beat in any remaining dry icing sugar from around the edges.
Beat with a spoon until smooth. Spread icing over slice. Refrigerate for a further 30 minutes or until icing has set. Cut into pieces. Serve