Preheat the oven to 350°F. Grease and lightly flour three 9" round cake pans.
1) For the layers: Whisk together the flour, baking soda, and salt; set aside. In a large mixing bowl, cream the butter for 30 seconds at medium speed. Pour in the sugar with the mixer running until it's all incorporated. Stop the mixer, scrape the sides and bottom of the bowl, then beat in the vanilla and the eggs, one at a time. Scrape the bowl again, beat in the peanut butter, then alternate adding the flour mixture and buttermilk, one third at a time. Scrape the bowl, beat for 30 seconds, then divide the dough between the three pans, using 2 1/4 cups or 18 ounces of batter per pan.
2) Bake for 25 to 30 minutes, until a tester inserted in the center comes out clean, and the edges of the cake just begin to pull away from the sides of the pan. Remove from the oven and cool in the pans on racks for 20 minutes before tipping the layers out of the pans and returning them to the racks to cool completely before assembling the cake.
3) For the filling: Cream together the butter, shortening, and confectioners' sugar until no lumps remain. Add the banana extract and baby food, and whip until the mixture is light and fluffy. Refrigerate until you're ready to build the cake.
4) For the frosting: Combine the butter, cooled chocolate, and vanilla. Add the confectioners' sugar and salt; beat until light and fluffy, adding milk as needed to adjust the consistency.
5) For the ganache: In a small saucepan or microwave-safe bowl, melt the chips and cream together, stirring until smooth. Use while still slightly warm.
6) To assemble the cake: Trim any domes off the tops of the cooled cake layers. Place a layer, cut side down, on a cake or serving plate; line the edges of the plate with strips of parchment. Spread the top of the layer with half the banana filling. Place another trimmed layer over the filling, cut side down, and spread it with the remaining filling. Place the third layer on top. Cover the top and sides of the cake with the chocolate frosting. Arrange the peanut butter cups around the top edge of the cake, and decorate the tops with piped peanut butter ganache (in the shape of a crown — for the king, of course!)