Why not just use real buttermilk for the Chocolate Sheet Cake instead of fiddling with milk and vinegar?
Nigella's Chocolate Sheet Cake recipe uses milk that has been soured with white wine or cider vinegar. This mimics buttermilk and the acidity is needed as bicarbonate of soda (baking soda) is used as the raising agent. As bicarbonate of soda is alkaline it needs an acid to react with to create bubbes of carbon dioxide that make the cake rise.
You could use store-bought buttermilk but Nigella opts to use soured milk for this recipe as the amount used is quite small (125ml/1/2 cup) and it saves having a half-used carton of buttermilk lingering in the fridge. In many countries buttermilk is not available or is not easy to find, plus using soured milk means that you can make the cake without having to make a special shopping trip if you don't have any buttermilk handy. Of course if you do have some buttermilk in the fridge then it would be perfect to use for the Chocolate Sheet Cake.