Add the pineapple juice and cornstarch to a small pot on medium high heat. Whisk occasionally until the juice thickens, about 10 minutes. Cool. Add the pineapple to the mixture and stir.
Fill frozen tart shells half full with pineapple mixture.
In a bowl, mix together sugar, egg, butter and coconut.
Place a tablespoonful of the mixture on top of the tarts.
Bake for 25 minutes or until light brown. Let cool before serving.