1. Preheat oven to 350°F (176°C) and prepare three 8 inch cake pans with non-stick baking spray and parchment paper in the bottom.
2. Combine the flour, cocoa, baking soda and salt in a medium sized bowl and set aside.
3. Add the butter, oil, sugar and vanilla extract to and large mixer bowl and beat together until light in color and fluffy, about 3-4 minutes. Do not skimp on the creaming time.
4. Add the eggs one at a time, mixing until mostly combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
5. Add half of the dry ingredients to the batter and mix until mostly combined.
6. Slowly add the milk and mix until well combined. The batter will look curdled, but that’s ok.
7. Add the remaining dry ingredients and mix until well combined and smooth. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated. Do not over mix the batter.
8. Divide the batter evenly between the prepared cake pans and bake for 19-21 minutes, or until a toothpick comes out with a few moist crumbs.
9. Remove cakes from the oven and allow to cool for 2-3 minutes, then remove to a cooling rack to finish cooling.
10. To make the frosting, beat together the butter, shortening and salt until smooth.
11. Slowly add 4 cups (460g) of powdered sugar and mix until smooth.
12. Add the vanilla extract and half of the water or milk and mix until smooth.
13. Add the other 5 cups (575g) of powdered sugar and mix until smooth.
14. Add the cocoa and mix until smooth.
15. Add the remaining water or milk until the frosting is the right consistency.
16. To build the cake, you’ll need something to be able to cut through the cake layers with. I used a 3 inch biscuit cutter, but a round cookie cutter, something similar, or a sharp knife would be fine too. Use your cutter to cut a hole all the way through one of the cake layers. You can the hole to be in the center of the cake.
17. For the other two cake layers, cut a hole only about half way through the cake layer and then use a spoon to carefully remove the cake within the marking.
18. Place one of the cake layers with only a partial hole onto a cake plate or cardboard cake circle and spread about 1/2 cup of frosting around the top of the cake, on the outside of the hole.
19. Place the next cake layer on top – the one with the full hole removed – then add another layer of frosting.
20. Spread a thin layer of frosting around the side of the hole of the cake, then fill the cake with your choice of candies and treats. You should fill the hole so that everything sticks up above the sides of the hole just a bit. The next layer of cake has a partial hole, so that extra filling/treats will fill in that space.
21. Add the final layer of cake on top, then frost the outside of the cake. Feel free to use my tutorial for frosting a smooth cake.
22. To make the ganache topping, add the chocolate chips to a medium sized bowl (see my tips for making chocolate ganache).
27. Add the heavy whipping cream to a microwave-safe measuring cup and microwave until it just begins to boil.
28. Pour the hot cream mixture over the chocolate chips and let it sit for 2-3 minutes, then whisk until smooth.
29. Drizzle the chocolate ganache around the edges of the cake, then fill in the center/top of the cake. See my tips on making a chocolate drip cake.
30. Use the remaining frosting to pipe swirls around the edge of the cake, then top the cake with additional candies.