1. Preheat oven to 350 degrees F.
2. Finely grind wafter cookies in a food processor. Drizzle in melted butter and pulse until evenly moistened. Dump into a lightly buttered 9-inch pie pan. Press firmly onto the sides and bottom (use a flat-bottomed glass or measuring cup to help compact the crust into an even layer).
3. Bake for 8 minutes just to set. Let cool completely.
4. To make filling, combine coconut milk, 1 1/4 cups whole milk, ube halaya, sugar, salt and cinnamon in a heavy saucepan set over medium heat. Bring to a simmer, stirring occasionally until sugar is dissolved.
5. Whisk remaining 1/4 cup milk with egg yolks in a medium bowl. Whisk in cornstarch until smooth and no lumps remain.
6. Slowly drizzle in a 1/4 cup or so of hot milk mixture, whisking vigorously. Continue to whisk in hot milk, 1/4 cup at a time, until about 1/3 of milk mixture has been incorporated and yolk mixture is warm to the touch.
7. Pour into saucepan with remaining milk mixture, and return to medium heat.
8. Bring to a boil, whisking frequently so the bottom of the custard doesn’t burn, and once it starts to bubble continue to cook for 2 minutes.
9. Remove from heat, and whisk in vanilla and ube extracts (optionally, if your mixture seems unusually chunky and you want a smoother custard, you can strain it through a fine mesh sieve to remove any large chunks.
10. Pour custard into cooled crust, spreading into an even layer. Let cool to room temperature, then cover and refrigerate at least 3 hours or overnight until completely set.
11. Just before serving, top with fresh whipped cream. Decorate with seasonal sprinkles, as desired.