Preheat oven to 350ºF (180ºC). Butter and flour 2 (9-inch) cake pans.
In a mixer fitted with a whisk attachment, cream the butter and shortening together. Add the sugar and continue to beat until the mixture is light and fluffy. Add the eggs one at a time, beating well after each addition. Next, add the vanilla.
In a medium mixing bowl whisk together the flour, baking powder, baking soda, and salt, mixing to combine.
With the mixer on low speed, slowly alternate adding the flour mixture and the buttermilk, starting with the flour, until all has been incorporated.
Divide the mixture between the two baking pans. Bake until the top of the cake is firm to the touch and a toothpick inserted into the center comes out clean, about 30 minutes.
Transfer the cakes to a wire rack and let cool in the pan for 10 minutes before turning the cakes out onto a wire rack. Let cool to the touch.
For the frosting:
Add the butter and powdered sugar to the bowl of a stand mixer. Beat the butter and powdered sugar together until the butter is light and fluffy, about 2 minutes.
Add the cocoa powder and vanilla and continue to beat, scrapping down the sides of the bowl occasionally, until the mixture is completely combined.
Add the heavy cream and continue to beat until light and fluffy. Taste the frosting and add additional powdered sugar as needed.
For the chocolate ganache:
Add the chocolate to a medium heat-proof mixing bowl. Add the cream to a small saucepan over medium heat. Bring the cream to a simmer then remove from heat.
Pour the cream over the chocolate and whisk until the mixture is smooth. Allow the chocolate ganache to cool slightly, about 10 minutes, before using.
For assembly:
Place one of the cake layers on a platter. Using a rubber spatula, spread 1/3 of the chocolate frosting over the cake, spreading to the edges. Top with the remaining cake layer. Spread the remaining frosting evenly over the top of the cake and the sides.
Pour the chocolate ganache onto the middle of the cake spreading it to the sides so that it just drips over the cake.
Chill the cake for about an hour then decorate the top of the cake with the candy until it has been covered.