Instructions

Make the crust

1. Preheat oven to 350°F. Set a 9-inch tart pan with a removable bottom on a sheet pan. Add the flour, cocoa powder, confectioners' sugar and salt to a medium mixing bowl and whisk together. Pour the melted butter and vanilla extract and stir until well mixed. Press the dough into the tart pan along the bottom and up the sides in an even layer.  
2. Bake until the crust goes from looking very wet to looking fairly dry, about 20 minutes. (Cocoa powder makes it hard to see when it's done--if your oven tends to run slow, give it a few more minutes, but not more than 25 or it's likely to burn.)
3. Let cool to room temperature before filling. (Usually takes 20-30 minutes.)

Make the Peanut Butter Mousse Filling

1. In the bowl of a stand mixer fitted with a whisk attachment (or a large mixing bowl if using a hand mixer) add the heavy cream and beat on medium high speed until it holds soft peaks, about 2-3 minutes. If using a stand mixer, transfer the cream to another mixing bowl, but no need to wash the mixer bowl.
2. Add the peanut butter, crème fraîche, sugar, vanilla, and salt to the bowl of your stand mixer (or another large mixing bowl if using a hand blender), and beat on medium speed until the mixture is uniform in color. Add a quarter cup of the whipped cream and beat on medium speed until it is just incorporated. Then remove the bowl from mixer, add the rest of the whipped cream and gently fold it into the peanut butter mixture with a rubber spatula until the mixture is all the same color.
3. Spread the peanut butter mousse into the chocolate tart crust in an even layer all the way to the edges. Refrigerate for at least an hour before adding the ganache.

Make the chocolate ganache topping

1. Add the chopped chocolate to a large heat-safe mixing bowl. Heat the heavy cream in a small saucepan just until it boils. Remove from the heat and immediately pour over the chocolate. Let sit for 30 seconds, then stir with a rubber spatula until the mixture is smooth.
2. Remove the tart from the refrigerator and carefully pour the ganache over the peanut butter filling in a circular motion to spread the ganache evenly over the top. Then carefully spread the ganache to the edges of the tart with a small offset spatula. Sprinkle with flaky sea salt (and decorate with chocolate pearls, if desired). Refrigerate until set, at least 2 hours or up to 3 days. Remove from the refrigerator, unmold the tart, and slice with a sharp knife. Serve cold.