Preheat oven to 350°F (175°C). Grease and line three 8-inch cake pans.
In a large bowl, whisk together flour, cocoa powder, baking powder, baking soda, salt, and both sugars.
Add eggs, buttermilk, vegetable oil, and vanilla. Mix until combined.
Slowly add hot coffee or boiling water and mix until smooth. Batter will be thin.
Divide evenly between prepared pans and bake for 28–32 minutes, or until a toothpick inserted comes out clean.
Cool in pans for 10 minutes, then transfer to wire racks to cool completely.
Beat butter until light and fluffy.
Add powdered sugar, cocoa powder, vanilla, salt, and cream. Beat until smooth and fluffy, adding more cream if needed.
Level cooled cake layers if needed.
Place the first cake layer on a cake stand. Spread with chocolate buttercream.
Repeat with remaining layers. Frost the entire cake with a smooth coat of buttercream.
Heat cream until steaming, pour over chocolate chips, let sit 1 minute.
Stir until smooth, let cool slightly.
Drizzle ganache over the top edge of the cake, letting it drip down the sides. Smooth a little ganache over the top center.
Pile candy corn and Halloween candies on top of the ganache.
Add extra candy or sprinkles around the base if desired.