Preheat oven @180° C for 10 minutes. Grease a cake tin with oil and line with parchment paper.
In a mixing bowl add in the milk and vinegar. Mix well and keep aside for 3-5 minutes. The milk will curdle and form buttermilk.
Add in sugar, oil and vanilla extract. Mix well to combine and incorporate the mixture together well.
Sieve in the all purpose flour, milk powder, baking powder, baking soda and salt. Sift them.
Gently fold them and make a smooth batter.
Now divide the batter into two portions.
Take the melted chocolate and add in the cocoa powder. Mix well to combine.
Add in mango puree in one of the portion and well until combine. Similarly add in the chocolate mixture in another portion and mix well until combine.
Take the prepared pan and start transferring the prepared batters onto the prepared pan alternately. Continue doing the same until the batter is completely over.
Tap the pan for 3-4 times to release any air bubbles trapped inside.
Take a skewer and swirl the batter randomly.
Bake them at a preheated oven @180° C for 30-35 minutes with both rods on. A skewer inserted should come out clean.
Remove from the oven and let it stand for about 5 minutes .
De-mould the cake and allow the cake to cool down completely. Slice and enjoy!