1. Set oven to 160C. Grease 24cm x 32cm Swiss roll pan; line base and long sides with baking paper, extending paper 5cm over sides.
2. Stir butter and chocolate in medium saucepan over low heat until smooth. Cool 10 minutes.
3. Meanwhile, drain pineapple well over medium bowl. Reserve 2 tablespoons juice.
4. Stir caster sugar, eggs, sifted flours, desiccated coconut and drained pineapple into chocolate mixture; spread mixture in pan. Bake about 35 minutes.
5. While slice cools in pan, sift icing sugar into medium heatproof bowl; stir in reserved pineapple juice. Set bowl over medium saucepan of simmering water; stir until icing is spreadable.
6. Spread icing sugar over slice; sprinkle with shredded coconut. Stand at room temperature until icing sets before cutting.
Will keep for a week in an airtight container.