Try this delicious recipe from River Cottage Australia.
For the base::
Line a slice pan with baking paper.
Finely crush your biscuits using a food processor. Stir in the coconut and zests until they are well mixed.
Place the butter and condensed milk into a saucepan over a low heat. Stir occasionally and then remove from the heat when the butter is melted and the mixture is smooth. Off the heat, stir the lemon/lime juice into the condensed milk mixture.
Add the condensed milk mix to the dry ingredients and combine thoroughly. The mixture will seem quite wet at this point.
Press the mixture into the prepared pan and smooth firmly using a small roller. Refrigerate.
For the icing::
Combine the icing sugar and lemon/lime juice in a small saucepan over a low heat. Stir until the mixture is warm (not hot) to touch and completely smooth.
Remove from heat and spread over cooled slice.
Finish with lemon/lime zest.
Refrigerate until cold and firm and then remove from pan, cut off edges and cut into pieces.
Store in an airtight container in the fridge or freezer.
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