Preheat oven to 325°F/160°C.
For the double boiler, fill a small saucepan with a few inches of water, set over low heat, set a bowl with the peanut butter and butter on top. The water should not touch the bottom of the bowl and should never be hotter than a simmer. Stir the mixture with a silicone spatula until the butter has melted and the mixture is completely combined, scraping down the sides as necessary and being careful not to incorporate air. Remove the top of the double boiler and set it aside.
Combine the sugar, cocoa powder, and salt in a large bowl and whisk to combine. Whisk in the milk, then eggs and vanilla. Stir in the peanut butter mixture.
Place the piecrust in a pie pan on a rimmed baking sheet. Dock the bottom of the piecrust with a fork and gently pour the filling into the crust and bake for 30 minutes. Rotate the baking sheet and bake for 30 minutes more, or until the filling is set and no longer jiggles. Transfer the pie to a wire rack and cool to room temperature, 30 minutes.
Set up the double boiler again and heat the cream and chocolate until melted, add the salt mix well.
Pour over the cooled pie and allow to cool completely for 2 hours.
