Cake: Preheat oven to 180 degrees C. Grease and flour a 20cm square pan.
In a large bowl, mix flour, sugar, pudding mix, baking powder and salt. Make a well in the centre and add copha, eggs, pineapple, 1/2 cup rum and vanilla. Beat until smooth. Stir in macadamia nuts.
Bake for 45 to 55 minutes, or until a toothpick inserted into cake comes out clean.
Glaze: Place water, butter and sugar in small saucepan. Boil 3 minutes. Remove from heat and add the rum.
To Serve: While cake is still very warm, poke holes all over top of cake with toothpick. Slowly spoon glaze onto cake. Cake will be very moist. Let stand for at least 15 minutes to absorb glaze. Best served while still warm.