In a large pot of water, add the closed can of condensed milk and let it simmer for 2-3 hrs. Make sure the can is submerged under water at all times. Once you take it out, let the can come to room temperature.
Pour out the contents of the can the rich dulce de leche into a bowl. Add the pumpkin spice vanilla and the pumpkin spice powder into the bowl and mix thoroughly.
Store in a clean bottle until needed.
Melt the chocolate, using free hand. Spoon melted chocolate in a tree shape on a parchment paper. Dust it lightly with cocoa powder and decorate it with the wafer paper leaves, edible sparkly sprinkles and sanding sugar.
Let it cool in the freezer until needed.
Melt the black candy melts and pour it in the silicone mold. Store it in the freezer until needed for decorating the cake
Preheat the oven to 350°F/ 180°C. Grease and line the cake tins with parchment paper.
In a bowl fitted with the paddle attached on a stand mixer. Add the butter, oil and sugars. Beat and cream the fats and sugars.
Add and beat one egg at a time. Add Buttermilk into the bowl.
Add the instant coffee powder to the hot water and keep aside.
Sift all the dry ingredients (AP flour, cocoa powder, baking powder and baking soda) into the bowl, mix on low speed while adding the hot coffee. Mix thoroughly until all the ingredients are incorporated
Pour the batter into all the prepared pans and bake for 20 (for the smallest) and 30-35 mins for the bigger ones. Remove from oven and let it cool on a cooling rack.
Add the butter and melted chocolate in a bowl fitted with the paddle attached on a stand mixer. Sift the cocoa powder and icing sugar in batches and beat for a couple of minutes.
Add the sour cream and whip the buttercream until smooth and silky.
I wanted the cakes layered in a staggering arrangement. I had to cut one of the 6" cake to stack up as i wanted.
Add a dollop of frosting on a cake board/ cake stand, place the larger layer on the bottom. Make a dam with the fudge frosting, add the spiced dulce de leche in the middle.
Repeat for all layers. Because the layers are smaller as they are stacked, mark the smaller cake on the larger one using a pick for making the dam for frosting.
The frosting coating need not be perfect, this is a halloween cake!
Decorate with the chocolate tree, candy skulls and chocolate dirt.