Preheat the oven to 180 degrees C. Butter 2-3 19cm round cake tins (I used two and filled one with 2/3rds of the mixture). Line with parchment paper, then butter and flour the pans.

Sift the flour, sugar, cocoa, baking soda, baking powder, and salt into the bowl of an electric mixer fitted with a paddle attachment and mix on low speed until combined. (I don't have a paddle attachment but my regular hand mixer worked fine) In another bowl, combine the buttermilk, oil, eggs, and vanilla. With the mixer on low speed, slowly add the wet ingredients to the dry.

With mixer still on low, add the coffee and stir just to combine, scraping the bottom of the bowl with a rubber spatula. Pour the batter into the prepared tins and bake for 35 to 40 minutes, until a cake tester comes out clean. Cool in the tins for 30 minutes, then turn them out onto a cooling rack and cool completely. (If you used two tins like me, wait until completely cool before slicing up cake into layers)
 

For the chocolate ganache:
300g dark chocolate (I used 70% cocoa Lindt)
150g milk chocolate
150g white chocolate
600ml pouring cream

Break up chocolate into small pieces and place in separate bowls. Slowly bring cream to just to the boil in a medium saucepan. Remove from the heat and cool for a couple minutes. Pour 300ml of cream over the dark chocolate, 150ml over the milk and 150ml over the white. Leave it to melt for a minute, then stir each of them to combine. Set aside to cool for 10 minutes. Using an electric mixer, beat each ganache separately until it is shiny, fluffy and just holds its shape (take care not to overmix or mixture will split, white chocolate especially). When ready to assemble, spread white chocolate ganache over bottom layer, cover with second layer of cake, then spread over milk chocolate ganache, top with final layer and cover entire cake with dark chocolate ganache. Make sure to work quickly or ganache will set. Serve at room temperature, run a knife under hot water before slicing up.

Optional: Use any leftover chocolate ganache to fill macaron shells and decorate on top of cake. See here for the recipe I used for the macaron shells.