Preheat oven to 350 degrees F. Grease and flour two 8-inch cake pans. Set aside.
In a medium mixing bowl, stir together the flour, cocoa powder, baking soda, baking powder, and salt until evenly combined. Set aside.
In a large mixing bowl, beat together vegetable oil, butter, and brown sugar until smooth, about 1 to 2 minutes. Add the eggs and vanilla, and beat until smooth. Scrape the sides of the bowl as needed.
Add half of the dry ingredients to the brown sugar mixture. Mix until just combined. Pour in all of the buttermilk and mix until evenly combined. Add the remaining dry ingredients and mix until combined. Scrape the sides of the bowl as needed. Add in the coffee and mix until smooth.
Pour half of the batter into each of the prepared pans.
Bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out clean. Remove cakes from oven and place on wire racks to cool completely.
Candied Bacon:
Preheat your oven to 400 degrees F. Line a baking sheet with aluminum foil.
Place the brown sugar and cayenne pepper in a shallow bowl or on a plate and stir together.
Dredge the bacon strips in the brown sugar mixture so they are well coated and place on the prepared baking sheet.
Bake for 10 minutes, flip and bake for another 7-10 minutes. (Watch the bacon closely after you flip it. It can go from almost done to OH NO! quickly) Remove from oven and let cool to room temperature.
Finely chop all the bacon. Reserve 1/3 of the bacon.
Chocolate Buttercream:
In a mixing bowl beat butter on low speed for 1 minute. Add 1 cup powdered sugar to the butter at a time, mixing between each addition until just combined. Scrape the sides of the bowl as needed. Once all the sugar is incorporated, mix on medium speed for 2 minutes.
Add the cocoa powder, vanilla, and heavy cream. Mix until combined and smooth, scraping the sides of the bowl as needed. Once smooth, beat on medium speed for 3 minutes until fluffy.
Assembly:
Portion about 2 cups of the chocolate buttercream into a separate bowl and mix in the other 2/3 of the chopped candied bacon.
Level out the tops of each cake, and then cut each layer in half horizontally so there are 4 even layers. Place one bottom half cake layer on a serving platter or cake stand. Portion 1/3 of the bacon chocolate buttercream onto the layer and spread evenly out to the edges. Add a layer of cake, 1/3 of the bacon buttercream, and spread the frosting. Repeat once more. Place the last cake layer on top, bottom side up to give the cake a nice clean edge on top.
Place 1/3 of the remaining chocolate buttercream in a piping bag fitted with a star tip. Set aside.
Frost the outside of the cake with the remaining chocolate buttercream so the cake is evenly coated on the sides and top.
Sprinkle the reserved bacon on top of the cake, leaving a 1-inch border around the edge.
Use the piping bag to make a border or rosettes around the outer edge of the cake top. Refrigerate until ready to serve.
Notes
If you don't have buttermilk on hand, you can substitute whole milk or 2% milk.
The coffee can be left out if desired and replace with more milk. The coffee helps boost the chocolate flavor of the cake.
Be sure to cook your bacon until it's nice and crispy! You want a textural element in the cake, not soft and chewy bacon.