First, make the cake: Preheat over to 350. Line three 9" round cake pans with parchment paper. Grease and flour sides.
Mix cake mix, eggs, oil, pudding and buttermilk in stand mixer with whisk attachment until smooth, about 5 minutes.
Divide batter between the 3 prepared pans.
Bake for 18-20 minutes, until toothpick inserted into middle of cake comes out with just a few crumbs.
Let cool in pans for 5 minutes.
Remove cakes to rack to finish cooling.
Next, make the peanut butter filling: Beat the peanut butter and vanilla together until smooth, about 2 minutes.
Add powdered sugar, about 1/2 cup at a time, beating well after each addition until mixture is smooth.
Add milk 1-2 tbsp at a time, beating well after each addition.
Beat at medium speed until smooth. Set aside.
Then, make your chocolate buttercream: Use stand mixed with paddle attachment to cream butter and shortening together until smooth.
Stir together powdered sugar, cocoa powder, and salt. Add to butter/shortening mixture ~1/2 cup at a time, mixing first at low, then medium, after each addition until smooth.
Add milk 1 tbsp at a time until desired consistency is achieved. Set aside.
Now, let's assemble: Level cakes. I used a Wilton cake leveler.
Spread half of peanut butter filling on top of one layer. Place next layer on top of filling. Spread rest of peanut butter filling on this layer. Place final cake layer on top. NOTE: I always put the top two cake layers top side down as I find it easier to frost the bottoms of cakes.
Apply a thin layer of chocolate butter cream around the cake. I usually put it back in the fridge for 5-10 minutes to let it seal up. After this, finish frosting with the remaining buttercream.
Time for some ganache: Place chips and syrup in heat proof bowl.
Heat heavy cream in microwave until it starts to boil (about 1 minute in my microwave).
Pour cream over chips and syrup.
Wrap bowl with plastic wrap and let sit for 5 minutes.
Remove plastic and whisk together.
Let sit for another 3-5 minutes, then pour over cake, starting in the middle of the top layer.
Use an offset spatula to take ganache to edges.
Finish up with some peanut butter buttercream: In stand mixer with paddle attachment, beat together butter, shortening, and peanut butter until smooth.
Add in powdered sugar, ~1/2 cup at at time, mixing well after each addition.
Add salt and mix well. Scrape sides and beat as needed.
Add cream, 1-2 tbsp at a time, beating well after each addition.
Beat on high for about 3 minutes, until buttercream has become light and fluffy. Scrape sides as needed.
Place in pasty bag and decorate as desired.