Preheat the oven to 340°F (fan).
Line and lightly grease an 8-inch baking tin.
Mix together the milk, orange juice, and vinegar set aside to thicken. Rub the orange zest into the sugar.
Once the milk has thickened, pour it into a large mixing bowl and add the remaining wet ingredients.
Using a sieve, sift in the dry ingredients.
Gently mix until just combined.
Separate the batter into two bowls.
Mix the melted chocolate and cocoa powder into one of the halves.
Layer the batter into the tin and bake for 30-40 minutes (or until well risen and cooked through it should spring back when touched and a toothpick inserted into the middle comes out clean).
Heat the cream until it begins to simmer, whisking constantly. Once it simmers, remove the pan from the heat and pour the cream over the chocolate. Leave it to melt for a couple of minutes before mixing.
Once cool, pour the ganache over the cake and leave it to set. Enjoy!