Prepare three 6” cake pans by lining with parchment paper and spraying with oil.
Preheat oven to 300 degrees.
Whisk hot coffee and both cocoa powders together until no lumps remain.
Add buttermilk, canola oil, egg, and vanilla. Whisk until combined.
In a separate bowl, whisk flour, sugar, baking soda, and salt together until combined.
Pour wet mixture into dry mixture and whisk until just combined. Do not overmix! Batter will be runny.
Pour into three prepared pans, weighing for accuracy (or eyeballing!).
If using cake strips, bake 28 mins. Without, bake 24 mins.
Check the cakes - tops should bounce back when lightly touched with your finger, and a toothpick inserted to the centre should come out with only a few crumbs - not wet.
Remove the cakes and let them cool in the pan for about 5 minutes, then invert onto a cooling rack to cool fully.
Make buttercream according to instructions (check the "Classic American Buttercream Recipe" page!)
Microwave the heavy cream in a bowl for 30 seconds, until hot.
Add in semi-sweet chocolate chips and let stand for about a minute, to soften up.
Stir until smooth and fully combined.
Once fully cooled, place first cake layer on a plate or serving dish. Spread a thin layer of frosting, to coat the top.
Fill a piping bag with buttercream, using the tip of your choice. I like to use a star tip as I find this holds in the filling the best.
Pipe a border around the edge of the cake.
Fill this ring in with ganache! It should be slightly cooled, so it's not too runny.
Place the next cake layer on top, and press down lightly.
Repeat!
Once all the layers are stacked, coat the cake in a thin layer of buttercream. Then place it in the fridge for at least 30 minutes, to allow the ganache and buttercream to set.
Wrap that baby up in a final coat of buttercream and place in the fridge to chill.
Re-heat your ganache, so it is a slightly runny consistency. You can test it by pouring a little bit down the side of your bowl.
Take the cake out of the fridge, and pour a small teaspoon of ganache on the top edge of the cake. Then push it over the edge, so it runs down the side.