Preheat oven to 350°F or 180°C.
Make buttermilk. Add vinegar in milk and keep it aside. Do not stir it.
Take about 1-2 tsp of flour and coat tutti frutti nicely in it. Tutti frutti will not sink to bottom.
Sift flour, cornstarch, baking powder and baking soda. Keep aside.
In a mixing bowl or stand mixer, add butter, oil and sugar with vanilla essence. Beat on medium speed till light and fluffy. You can also use hand-held electric beater or your hand beater.
Add yogurt/curd and beat again till it is nicely mixed.
Add flour mix and buttermilk alternately in 3 parts. Do not overbeat the batter, just mix lightly.
Now add tutti frutti and mix lightly in batter. You can reserve 1-2 tsp and sprinkle on top.
Bake in preheated oven for about 40-45 min at 350°F or 180°C.
Check for doneness after 45 mins. Insert a tooth pick in the middle. If it comes out clean or with small crumbs, cake is baked. If some batter is still sticking to it, cover with an aluminum foil. Bake for another 10 mins.
Once done, take out the cake carefully and keep on cooling rack.
Cut when cake is completely cool.