Preheat the oven to 175 C. Grease a 9-inch springform tin and set aside.
To make the batter, first combine the milk and lemon juice in a small bowl, stir and set aside to let it curdle.
In a mixing bowl, whisk together the oil and butter, then add the sugar and vanilla and whisk again till combined.
Sift in the flour, cocoa, baking soda, baking powder and salt. Fold gently to combine, then add the soured milk a little at a time, continuing to fold it all together with a spatula. The batter is smooth and pourable, but not runny.
Finally, stir in the chocolate chips (feel free to add more!). Transfer the batter to the prepared tin and smoothen the top. Bake for 45 to 50 minutes until a toothpick poked in the center comes out clean. Set aside to cool completely.
While the cake is cooling, make the ganache for the frosting. Add the chopped chocolate to a heatproof bowl. Then in a small saucepan, heat the cream on low until hot but not simmering. Pour over the chocolate and let it rest for a few minutes. Then stir gently with a spatula to melt the chocolate completely and you'll have a smooth, runny ganache. See photos below.
Refrigerate the ganache for 1 hour. When ready to assemble the cake, whip the ganache with the powdered sugar and vanilla, using a hand mixer on medium-high speed for 3 to 4 minutes until soft peaks form and the ganache is now pale, thick and fluffy. Keep refrigerated while you slice the cake.
Remove the sides of the springform pan (I leave the cake on the base of the pan itself, but you can transfer it if you wish to a serving platter using a large metal spatula). Slice the cake into two horizontally. Gently lift the top half off and place on a plate nearby.
Spread about half of the whipped ganache all over the bottom half of the cake, in an even layer. Carefully lift the top half and place it over the ganache layer and press down gently.
Cover the top and sides with the rest of the ganache. I like to leave the sides lightly frosted for more of a 'naked cake' look. For the chocolate sauce decoration on top, see notes below.
You can slice the cake immediately after frosting. Otherwise, store in the refrigerator, covered and then bring to room temperature before slicing so the frosting and cake both soften.
Store individual slices in an airtight tin in the fridge for 5 to 6 days. Happy baking!