Makes one 9-inch (23 cm) pie.
Peanut Butter Mousse
For the cookie crust, cream the butter and brown sugar in a large mixing bowl until smooth. Beat in the peanut butter, followed by the egg and vanilla. Sift in the flour, cocoa powder, baking powder and salt and stir until combined. The dough can be rolled right away, or wrapped and chilled for rolling later.
Lightly knead the dough to soften it and then roll it out on a lightly floured surface to a circle that is about 1/4 inch (6 mm) thick. Grease a 9-inch (23 cm) pie plate and line it with the pastry. Trim away any excess dough and pinch the edges. Chill this for 30 minutes. Preheat the oven to 350ºF (180ºC). Dock the bottom of the pie shell with a fork and bake for about 20 minutes, until the bottom of the shell loses its shine. Cool the shell completely on a rack before filling.
For the ganache, heat the cream in a small saucepan over high heat until it just begins to bubble (watch it carefully, as it can boil over). Pour this over the chopped chocolate in a small bowl, let it sit for a few seconds and then stir until the mixture is smooth and the chocolate has fully melted. Sprinkle the chopped peanuts in an even layer over the crust and then pour the in ganache and swirl the crust so that it fully coats the bottom. Chill this while preparing the mousse.
Whip the cream on high speed using electric beaters of a stand mixer fitted with the whip attachment so that it holds a soft peak when the beaters are lifted and then chill. Again, using beaters or the stand mixer with the whip attachment, whip the cream cheese and peanut butter together until smooth. Add the icing sugar and beat (starting on low speed and then increasing) until smooth. Stir in the vanilla. Fold in the whipped cream in two additions, until evenly incorporated. Spoon this into the crust and use your spatula to either smooth out or swirl the top of the mousse (there will be too much mousse to make it level). Use a vegetable peeler to grate chocolate shavings over the top of the pie and chill for at least 2 hours before serving.
The pie will keep, refrigerated, for up to 3 days.