melt butter in microwave in a bowl – 40 seconds on high (or as needed)  add sweetened condensed milk and stir  until smooth.
Put a packet of plain cookies in a sturdy plastic bag. With your hands pat the bag until the cookies crumble.

Put crumbs, lemon juice and coconut in a bowl and mix well. Press into a 23 cm square tray (9 inches square) Refrigerate overnight before slicing or the squares will come out sticky.