Preheat oven to 350 degrees and spray 3 9 inch round cake pans with Pam baking spray. Set aside.
In a large mixing bowl combine all cake ingredients together until fully combined.
Divide batter between the 3 baking pans and bake for 25-30 minutes or until a toothpick inserted in the center comes out clean. Cool completely.
Once cooled, remove cakes from cake pan and cut the top domes off so they are flat.
For the Peanut Butter Buttercream
In a large bowl, cream butter, powdered sugar, peanut butter ,vanilla and salt. Stir in powdered sugar a little at a time. Add heavy cream until you reach your desired consistency.
For the Ganache
In a small saucepan, heat heavy whipping cream until steaming. While heavy whipping cream is heating up, place chocolate chips into a heat-proof bowl.
Once the heavy whipping cream is heated, pour over the chocolate chips.Allow to sit for 1 minute before whisking until smooth.
Pour ganache into a squeeze bottle.
Assemble
Place the first cake layer onto a serving plate. Scoop 1 cup of frosting onto the first layer and spread it out. Repeat steps with remaining cake layers. Frost top and sides too.Scoop remaining frosting into a piping bag with a star tip ,and set aside
Using the squeeze bottle, to create a drizzle effect around the top of the cake. Squeeze some ganache over the side allow the chocolate ganache to drip down.Pour remaining ganache onto the center of the top of the cake and smooth evenly to coat the top.
Allow to sit for 10 minutes to allow ganache to harden.Pipe dollops of frosting onto the top cake.
Take the potato peeler and run it along the edge of the chocolate bar like you're peeling an apple to create shavings.Place the Reese Peanut Butter Cups in between the frosting dollops.
Notes
Ingredient Notes and Shopping Tips
Chocolate Cake Mix: I'm partial to Betty Crocker's chocolate fudge, but feel free to use your favorite.
Milk: Use whole milk instead of the water called for on the box for the moistest cake.
Eggs: Use room temperature eggs; they'll incorporate more smoothly.
Butter: Use butter instead of the oil called for on the box. It adds a wonderful flavor and richness that oil just can't provide.
Peanut Butter: I used Jif. Use regular, not natural peanut butter, which can cause your buttercream to split.
Heavy Cream: You can also use milk if that's what you have on hand.
Tips
Weigh Batter: Use a food scale to get perfectly even layers with no guesswork.
NonStick: Line your cake pans with parchment paper for easy removal every time.
Get Flat Layers: To ensure to cake won't fall or slide apart, cut off cake domes with a cake leveler or sharp bread knife.
How To Tell When Cake Is Done: Insert a toothpick near the center. If it comes out clean or with just a few crumbs, it's done.
Do a Crumb Coat: Use around 1 cup of frosting and thinly frost the outside of the cake. Scrape off any extra with a bench scraper. Discard or save this for graham crackers as we don't want that frosting going back on our cake.
Super Smooth Frosting: Frost with an offset spatula and revolving cake stand. Spin and smooth until you have a nice even layer.