Preheat your oven to 170°C. Grease the cake pans with butter and dust them with flour to prevent sticking.
Make the buttermilk by combining vinegar with milk. Let the mixture rest for 10 minutes.
In a bowl, add the dry ingredients: flour, baking powder, baking soda, cocoa powder, and salt. Stir with a whisk to combine evenly.
Add the granulated sugar, brown sugar, egg, buttermilk, oil and vanilla extract into the dry ingredients and mix until combined.
Gradually pour the boiling water into the batter while whisking and mixing until everything is fully combined.
Pour the batter into the cake pans and bake for 20 mins or until a test toothpick comes out clean. Let the cake layers cool before assembling.
Make the dark chocolate ganache: heat the cream until it almost starts boiling and pour it over the dark chocolate (placed in a heat-proof bowl). Let the cream sit for a minute, allowing the chocolate to melt, then whisk until the chocolate is fully melted and combined with the cream.
Make the milk chocolate ganache: follow the same steps as the dark chocolate ganache.