To make the crust, stir together the flour, sugar and salt in a large bowl. Add the butter and toss to coat the butter. Use a fork or pastry blender to cut the butter into the dry ingredients until the butter is the size of small peas.
Create a well in the middle. Add the buttermilk into the well and use your fingers or a wooden spoon to combine. Add a more buttermilk a tablespoon at a time, if necessary, but the mixture should be a little on the dry side.
Dump the contents back onto your floured surface and form into a disk. Wrap in plastic wrap and chill in the fridge for one hour. Can be made in advance.
On a lightly floured surface, roll the pie crust into a 13 inch circle. Move to a 9-inch pie dish, trimming and folding down the edges. Chill in the fridge for 1 hour.
Preheat oven to 400°F. Grease a sheet of foil and press into the pie dish, greased side down. Make sure all of the crust is covered. Fill the foiled pie dish with pie weights or dried beans.
Bake for 20 minutes, remove the foil and weights and bake for an additional 10 minutes, or until golden brown. Cool completely.
To make the filling, reduce oven temperature to 350°F. Spread the coconut on a sheet pan and toast in the oven in 2 minute intervals, stirring frequently, until evenly toasted. Be careful, the coconut can burn quickly. Let cool.
In a medium saucepan, combine 2 cups of the milk and sugar and heat over medium high heat until steaming and sugar is dissolved (do not boil).
Meanwhile, whisk together the egg yolks in a large bowl. Slowly add the heated milk mixture to the yolks while whisking constantly. Combine the remaining ¼ cup milk and cornstarch and whisk until no lumps remain, and add to the egg mixture.
Return to saucepan and cook over medium heat until thickened, stirring constantly. Remove from heat and add the vanilla extract, butter, and toasted coconut.
Spoon the filling into the baked pie shell and chill in the fridge for at least one hour. To make the topping, beat the cream until thickened. Add the vanilla extract and sugar and beat until just combined. Spread on top of the filling. Sprinkle with additional coconut. Chill until ready to serve.
The pie will keep in the fridge for several days.