Make the pie crust. Add the flour, butter, salt and sugar to a food processor and pulse a few times until the butter looks like small peas. With the motor running, slowly add the cold water, and pulse together until a ball of dough forms.
Dive dough in half and wrap in plastic wrap, forming a smooth round disk. Place dough in fridge for at least 20 minutes (or freezer for 10-15 minutes). This will be your crusts for both pies.
Make the sweet potato filling. Boil purple sweet potatoes potatoes for about 20 minutes until the potatoes are very soft. Then pour water out and add potatoes to (clean) food processor and pulse a few times to puree potatoes.
Add melted butter, evaporated milk, eggs, sugar and flavorings. Process everything together until mixture becomes a smooth creamy consistency. Set aside.
Roll out pie crust. Once pie crust is chilled, roll out to about ½ an inch wider than the pie dish you'll be using. Roll out on a floured surface with a floured rolling pin and fit into a 9 inch pie dish. Crimp the edge of pie into desired pattern. Repeat with other dough if making 2 pies.
Bake sweet potato pie. Pour sweet potato filling into pie shells and bake at 350 degrees Fahrenheit for 40-45 minutes. You will know pie is done when the crust is lightly golden brown and you can insert a knife into the potato layer and it comes out clean. If crust browns too fast, cover with foil and continue baking.
Cool pie to room temperature. Once pie is done remove from oven and allow to cool to room temperature for a few hours. Or place in fridge overnight.
Make the haupia layer. Mix together the sugar and cornstarch in a bowl and set aside. Heat coconut milk and water in a small pot until warm and add the sugar and cornstarch mixture. Whisk together and continue whisking for 5-8 minutes until sugar dissolves and haupia becomes thick, almost the consistency of thick honey. Once ready, pour haupia over sweet potato layer and top with chopped macadamia nuts and shredded coconut.
Allow pie to set and chill for several hours until haupia layer is cool and serve.