1. Preheat oven to 325 degrees F. To make the shortbread crust, mix together the flour, powdered sugar, and melted margarine. Press this mixture into a 9" pie pan, using your fingers to push it out to the edges and up the sides. Poke this crust with a fork a few times to prevent poofy bubbles. Bake for 20 to 25 minutes, or until the crust is slightly browning.
2. For filling, sift the cornstarch into a saucepot. Add about 1/2 the coconut milk and whisk until smooth before adding the rest of the coconut milk, water, and sugar. Add lemon zest and lemon juice. Bring the mixture to a boil slowly over medium-high heat, whisking frequently until thickened, 10 to 15 minutes.
3. Pour the filling into the pre-baked pie crust and let set for 5 to 10 minutes at room temperature before refrigerating until fully cooled (a few hours). The filling should set up almost like jello.
4. For garnish, toast the coconut flakes lightly to garnish the pie, or add a few curls of lemon peel. I quick-candied my lemon curls by dipping them into a hot simple syrup and then sprinkling granulated sugar on them.
Source of recipe: I modified this from the traditional Hawaiin haupia recipe.