Special equipment: a jumbo silicone cupcake mold; a silicone pastry brush
For the cake: Preheat the oven to 175°C. Prepare the cake batter according to the package instructions. Pour into the bottom of a jumbo silicone cupcake mold and bake until a tester comes out clean, about 1 hour. Let cool completely, then unmold the cake onto a cooling rack set in a baking sheet.
For the filling: Meanwhile, beat together the peanut butter and butter with an electric mixer until completely combined. Slowly add in the icing sugar about a 1/4 cup at a time, beating in between additions, until completely incorporated. Beat in the salt. Set aside until ready to use.
Cut off the domed top of the cake if one has formed, and save for another use (or eat!). Cut the cake horizontally into 3 layers and put them on a work surface Use a paring knife to cut out a 3 1/2-inch hole from the middle layer; set the cake layer aside (save the cutout for another use).
Stack the bottom and middle cake layers on the prepared cooling rack. Fill the hole in the center with the peanut butter filling. Place the last cake layer on top. Invert the cake onto a plate and chill in the freezer for 10 minutes.
Pour some of the hardening chocolate sauce over the top of the cake and use a rubber spatula to gently smooth and level it. Using a silicone brush, apply more of the chocolate sauce on the sides of the cake. Chill in the refrigerator for 10 minutes, then invert the cake onto a serving platter. Spread the remaining chocolate sauce on top of the cake in an even layer and chill in the refrigerator until set.
Cut the cake into slices, exposing the peanut butter center.
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