Preheat the oven to 180°C and grease and line the base of a slice tin around 18cm x 26cm or similar.
Cream butter and first measure of sugar with an electric beater until pale and fluffy, add vanilla and beat well. Using a spatula, stir in flour and desiccated coconut to form a soft dough. Divide mixture into thirds, then combine two of the quantities to make one large and one small portion.
Press the larger amount into the base of the lined slice tin and smooth with wet fingertips of the back of a wet spoon. Bake for 10-12 minutes until just starting to turn golden. Add the thread coconut to the remaining dough and mix well, then chill in the fridge for 10 minutes.
Toss feijoas with the second measure of sugar and cinnamon. Arrange evenly over the partially cooked base, then crumble over the coconut dough.
Bake for 25-30 minutes until golden and puffed. Dust with icing sugar and serve warm with ice cream or allow to cool and slice.