Step 1

Place custard powder in a medium saucepan and gradually add milk, whisking, to form a smooth paste. Whisk in remaining milk and cream. Add sugar, zest and juice and whisk over gentle heat until just starting to thicken, being careful not to let it boil or catch on the bottom of the saucepan.

Step 2

Place boiling water in a small bowl and sprinkle over gelatine. Stir with a fork to dissolve. Add to custard and whisk well to combine. Set aside to cool and thicken.

Step 3

Line an 18cm x 27cm slice tin with baking paper, leaving plenty overhanging to make it easy to remove the finished slice from the tin. Place SAO biscuits face side down in a single layer. You will need to cut some to fit snuggly. When custard mixture has cooled to a spreadable consistency, spoon carefully on top of biscuits and spread into an even layer. Top with more SAO biscuits. Refrigerate for 1 hour to set.

Step 4

To make icing, place icing sugar in a bowl and whisk in lemon juice gradually until you reach a spreadable consistency. Spread over slice in tin, sprinkle with extra lemon zest and refrigerate for 10 minutes to set.

Step 5

Using overhanging baking paper, gently lift slice out of tin. Use a sharp knife to cut into squares or rectangles to serve.