Preheat oven to 300.
In large bowl whisk together your dry ingredients until combined.
Add in your eggs, buttermilk, oil, vanilla and water and stir until combined well.
Line 3 8" baking pans with parchment paper and spray with non-stick cooking spray.
Pour cake batter evenly among the 3 pans.
Bake for about 30 minutes or until center us set and toothpick comes out clean.
Remove from oven and let cool in pans for about 5 minutes before turning out onto cooking racks to cool completely.
To make your frosting in large bowl with stand or hand mixer mix together your butter and peanut butter until smooth.
Add in your vanilla and mix until blended.
Add in your powdered sugar one cup at a time until you reach your desired consistency.
Add in heavy cream if it gets to thick.
To assemble place one of your cake layers on a cake stand or turn table and cover with about 3/4 cup of your frosting. Repeat layers.
Spread a fine layer of frosting on the outside of the cake to create a crumb coat and refrigerate for about 15-20 minutes to set.
Remove from refrigerator and finish frosting top and outside of cake, reserving about 1 cup of frosting for the top.
Place back in refrigerator while making your ganache.
To make ganache place your chocolate chips in a heat safe bowl.
Heat your whipping cream on stove or in microwave until simmering.
Pour over your chocolate chips and cover and let sit for about 5 minutes.
Remove cover and whisk together until smooth, let sit to come to room temp.
Once room temp remove your cake from the refrigerator and start spooning or piping drops of ganache down the side, the cooled cake will help the ganache set up fast.
Keep going all around the cake.
Then spoon the ganache in center of the cake and spread to the sides with an offset spatula.
Let set up for about 10 minutes.
Pipe remains frosting in swirls around the top of your cake and place peanut butter cups on swirls and sprinkle with chocolate sprinkles if desired.