Preheat the oven to 190℃ (180℃ fan)/375°F/gas mark 5. Line a 25x38 cm (10x15in) Swiss roll tin with baking parchment so the parchment rises about 2cm (1in) above the sides of the tin.
Place the egg whites in a large bowl and whisk until standing in soft peaks. Add the caster sugar a spoonful at a time whisking all the time. Continue to whisk until you have a stiff glossy meringue.
Whisk in the vinegar and cornflour. Sprinkle the coconut, coconut extract, if using, over the surface of the meringue and fold in
Spoon the meringue into the prepared tin and spread level. Bake for 30 minutes until the meringue is pale golden.
Remove from the oven and allow to cool completely in the tin. If making the day before cover with a clean tea towel and set aside.
When ready to assemble. Cut the pineapple into thin wedges. Cut away the peel and the tough centre core and discard. Cut one of the wedges into 10 slices and set aside. Roughly chop the remaining pineapple into small bite size pieces
Whip the cream and rum together until standing in soft peaks. Take care not to over whip or it will be hard to spread over the meringue.
Place a sheet of parchment on the work surface and turn out the meringue on top of the parchment. Carefully peel away the lining paper and spread most of the cream over the surface of the meringue, reserving a little for decoration.
Spread the chopped pineapple evenly over the surface of the cream using the parchment to help you roll up the meringue like a Swiss Roll and transfer to a serving plate, seam side down.
Spoon or pipe the remaining cream down the centre of the roll and use the reserved pineapple slices. Toast a little extra coconut until golden and sprinkle over the roll. Serve
Best eaten the day it is assembled.Not suitable for freezing.