1.Preheat oven to 220°C (200°C fan-forced). Line a 10x20cm lamington tray or 23cm square cake tin with plastic wrap. Place pineapple on a roasting tray and roast until skin is black, about 90mins. when cool, remove skin with a seated knife. Remove and discard core, then use a stick blender to puttee flesh until smooth. Set aside.
2.Place caster sugar and ½ cup water in a large saucepan. Stir to dissolve and bring to the boil, brushing down sides with a wet pastry brush from time to time. Simmer until mixture reaches 115°C on a confectionery thermometer then add tartaric acid and lemon juice.
3.Meanwhile, place icing sugar, cornflour and ½ cup cold tap water in a bowl and stir to combine. Add cornflour mixture and 2 cups boiling water to hot sugar syrup. Cook over medium heat, stirring with a whisk, until thick and translucent, about 15 mins.
4.Soak gelatin in cold water until soft, about 2mins. Squeeze out excess water then stir gelatin through mixture. Add pineapple puree and stir until combined. Pour into lined tray and refrigerate 2 hrs or until set. Turn onto a board and cut into squares with a slightly wet knife.
Makes about 24Note