Make the sauce by placing its butter and sugar in a small pot over medium heat. Cook and stir until the sugar melts. Add the rum and simmer 1 minute. Stir in cream and vanilla and simmer 2 minutes more, until lightly thickened. Remove sauce from the heat and cool. Transfer to a tight-sealing jar and refrigerate sauce until needed.
Preheat the oven to 350°F. Carefully line the piecrust with foil, and then fill with pie weights, raw rice or dried beans (see Note). Bake the crust for 30 minutes. Remove the foil and weights and bake 15 minutes more, or until the crust is light golden and cooked. Set the piecrust on a baking rack and cool.
Beat the eggs for the filling in a bowl until well blended. Mix in 1 cup of the milk, and then set aside.
Combine the sugar, flour and salt in a heavy-bottomed pot. Add the 1 cup of remaining milk and whisk until smooth.
Set the pot over medium heat. Bring to a simmer, whisking constantly, and cook until mixture is very thick, about 2 minutes, and then remove from heat.
Temper the cold egg/milk mixture with 1/2 cup of the hot mixture, by very slowing whisking the latter in. Slowly whisk the tempered egg mixture into the pot with the remaining hot mixture until well combined.
Place the pot back on the heat and mix in the 1 cup coconut. Bring to a simmer, stirring constantly, and simmer 2 to 3 minutes, until the mixture is quite thick, but still spreadable.
Remove from the heat, stir in butter and vanilla, and let sit 2 minutes. Now pour and spread the filling into the piecrust. Cool filled pie on a baking rack to room temperature. (Filling should set once cooled; if not, refrigerate the pie to set.)
When ready to serve, take the butterscotch sauce out of the refrigerator and heat in the microwave a few seconds to make fluid again. Top and spread or pipe the entire pie with whipped cream. Sprinkle the pie with the toasted coconut, drizzle with some of the butterscotch sauce, and served the rest alongside the pie.
Note: Pie weights are sold at stores selling a good selection of kitchenware. If you use rice or beans for blind baking the piecrust, you’ll need about 2 cups. After using, cool them to room temperature, place them in a bag and store them away until you need to blind bake a piecrust again.
To toast coconut, spread on a non-stick baking and cook in 350˚F for 4 to 5 minutes, until lightly toasted.
For a recipe to make you own piecrust, search Flaky Pie Dough at thriftyfoods.com/recipes.
Recipe Options: Although the pie won't be as sinfully rich, you can certainly serve it without the butterscotch rum sauce and still be very happy.