Instructions

1. Heat the 3/4 cup heavy cream to a near boil in a medium saucepan. Add the chocolate chips and the 1/2 teaspoon vanilla. Immediately remove from the heat and set aside for 5 minutes, then whisk until smooth. Pour a little more than half of the chocolate into the middle of the pie shell. Immediately tilt the pan this way and that, so the chocolate covers the bottom and runs partway up the sides. Set aside on a rack to cool. 
2. Combine the sugar, cornstarch, and salt in a medium-size saucepan (not aluminum), preferably one with a heavy bottom. Whisk to stir. Whisk in the milk and yolks. Place over medium heat and cook, whisking virtually nonstop, until the mixture starts to thicken and comes to a boil, about 5 to 7 minutes. (You may have to pause your whisking for 3 to 4 seconds to see if it's actually boiling.) When it reaches a boil, lower the heat slightly and continue to heat and whisk nonstop for 1 1/2 minutes.
3. Immediately remove from the heat and stir in the 2 teaspoons vanilla, the peanut butter chips, and the butter. Wait 3 to 4 minutes, then continue whisking until smooth. Pour about two-thirds of this peanut butter cream over the chocolate and smooth it out with a spoon. Transfer the remaining peanut butter cream to a small bowl and press a piece of plastic wrap directly over it to prevent a skin from forming. Cool the pie briefly, then transfer to the refrigerator and chill for at least 2 hours, preferably longer.
4. For the final assembly, chill a bowl and your electric mixer's beaters for about 10 minutes. Pour the heavy cream into the chilled bowl and beat well. When it starts to mound slightly, add the confectioners' sugar and vanilla and continue to beat until incorporated. Add the remaining peanut butter cream and continue to beat until the whipped cream is firm; don't overdo it or it will turn grainy. Smooth the topping over the pie, then refrigerate until serving. Serve slices garnished with chopped peanuts and the (rewarmed) leftover chocolate, if desired.