Butter a 9-inch square metal baking pan. Add a layer of parchment paper, leaving the edges long enough to lift the bars out of the pan for easier slicing. Butter the parchment paper, pressing it to adhere smoothly to the pan.
To make the chocolate shortbread, mix butter, cocoa, nibs, sugar, salt, and vanilla in a medium bowl. Add flour and mix until blended. Press the dough into an even layer over the bottom of the prepared pan. Generously prick the dough 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 pan 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. Tilt and rotate the pan to spread the ganache to the edges. Sprinkle chopped peanuts over the top.
Refrigerate uncovered for several hours before slicing. Serve cold or at room temperature.