To prepare the crust: Preheat oven to 350° F. Mix the macadamia nuts, graham crackers and melted butter. Pat the crust into a 9-inch springform pan. Bake for 10 minutes in 350° F oven. Remove the pan from the oven, but leave the oven on.
To prepare the cheesecake: Put the mashed sweet potato, cream cheese, eggs, ¾ cup sugar and vanilla in a mixer bowl and beat until well blended. Pour the cheesecake batter over the crust in the springform pan. Bake for 1 hour in the 350° F oven. Cool completely.
To prepare the haupia: Mix together the coconut milk, ¼ cup sugar, water and cornstarch in a heavy saucepan. Cook over low heat, stirring constantly, until the haupia thickens.
To assemble: Spread the warm haupia evenly over the top of the cheesecake.