Line a slice tin with baking paper, leaving a little overhang to make removing the slice easier.
Crush the biscuits into fine crumbs using a food processor or rolling pin.
Place the crushed biscuits and coconut into a large mixing bowl and stir to combine.
Gently melt the 100g butter, then stir through the sweetened condensed milk and 2 Tbsp lemon juice.
Pour the butter mixture over the biscuit crumbs and mix until well combined.
Press the mixture firmly and evenly into the prepared slice tin. Refrigerate while preparing the icing.
To make the icing, place the softened butter, icing sugar and 1 Tbsp lemon juice into a mixing bowl. Beat for approximately 2 minutes until smooth and creamy.
Spread the lemon icing evenly over the biscuit base.
Refrigerate until firm, then cut into squares or slices to serve.