Preheat oven to 160°C. Grease a 3cmdeep, 20cm x 30cm (base) slab pan. Line base and sides with baking paper, allowing a 2cm overhang at both long ends.
Combine muesli, dried cranberries and coconut in a bowl. Make a well in centre of mixture. Place butter and sugar in a saucepan over low heat. Cook, stirring, for 2 minutes or until butter is melted and sugar dissolved. Add butter mixture to muesli mixture. Stir to combine.
Spoon mixture into prepared pan. Using the back of a spoon, press mixture into pan. Bake for 20 minutes or until golden and firm. Allow to cool in pan.
Make lemon icing: Place icing sugar, butter, lemon juice and 1 teaspoon cold water in a bowl. Beat with a wooden spoon until smooth. Spread icing over slice. Refrigerate for 30 minutes or until icing has set. Lift out slice using baking paper. Cut into pieces. Serve.