I used Duncan Hines brand cake mix. If you are using cake mix like me, bake the cake according to package instructions.
1 cup buttermilk = 1 tablespoon lemon juice or white vinegar + enough milk to make 1 cup. Let stand for 5 minutes
I had some left over cake batter and made 3 std size cupcakes with them. So if you want you can try this as cupcake as well, but making it as a single cake is easier and less messier.
I used Philadelphia Full Fat cream cheese spread for icing, I used butter at room temp and cold cheese to make icing. Also I used only 2.5 cups of icing sugar and 1 tbsp vanilla essence.
You can make the cake 2 days ahead and cover it with cling film and refrigerate. Though the cake is pretty moist straight out of the fridge, I kept it at room temp for 20-25 mins, before serving.