Sieve flour. Add butter and flour together till the texture resembles that of breadcrumbs. Add baking powder and sugar and mix lightly. 

Divide the milk into two parts. To one part add vinegar. Warm the other part of milk slightly and add soda. Now mix both the parts together. The milk will start foaming (it will be in form of bubbles). Add this to the cake dough very quickly. Mix fast and well. Keep aside. 

Take a cake tin and grease it with butter. Put butter paper according to its size and dust lightly with flour. Bake it in a preheated oven at 190C/375F for 20 minutes. Then lower the temperature to 150C/300F for another 20 minutes. If a knife inserted in the center comes out clean, remove from the oven after 5 minutes.