To make the crust, mix butter, cocoa, nibs, sugar, salt, and vanilla in a medium bowl. Add flour and mix until blended. Press the dough evenly over the bottom and up the sides of a 10 or 11-inch tart pan with a removable bottom. Generously prick the bottom and sides of the crust with the tines of a fork. Place in the lower third of an oven preheated to 350° F and bake until the crust feels firm to the touch, about 20 to 25 minutes. Cool completely.
For the filling, cream together peanut butter, butter, vanilla, and salt. Blend in confectioners sugar, mixing well to combine. Add the cream and beat until the filling is smooth and spreadable.
Scrape the filling into the cooled tart crust and spread it evenly to the edges, smoothing the top with an offset spatula.
To prepare the ganache, place chopped chocolate in a measuring glass with a pour spout; set aside. Bring cream to a boil over medium heat. Pour hot cream over the chocolate. Without stirring, let the cream-chocolate mixture sit for 2 minutes. After 2 minutes, stir slowly with a rubber spatula until the mixture is smooth and glossy.
Acting quickly, pour ganache over the peanut butter filling. Carefully pick up the tart pan, tilting and rotating to spread the ganache to the tart’s edges as needed. Sprinkle chopped peanuts over the ganache.
Refrigerate uncovered until set, about 30 minutes. Served chilled or at room temperature.