Preheat the oven to 300°F Prick the frozen pie shells all over with a fork and partially bake the shells for about 8 minutes, until they start to puff slightly.
Meanwhile, place the egg, yolks, sugars, salt, buttermilk, cream and butter in a stainless-stell mixing bowl. Set the mixing bowl over a pot of boiling water. Whisk the mixture constantly, until no yello streaks remain and the mixture is shiny and warm to the touch, about 3 or 4 minutes. Stir in the coconut and pour the filling into the crusts. Bake until golden and firm, 50 to 65 minutes. Makes 2 pies, with 12 to 16 servings total.