In a bowl, combine yogurt/dahi and baking soda. Let it rest for a minute and you’ll see that the mixture will become voluminous.
Now add in the sugar, sunflower oil and milk in the same bowl and whisk until combined.
Sift in the dry ingredients- all purpose flour, cocoa powder and baking powder. Mix everything together to form a lump free batter. Do not overmix at this stage.
Transfer the batter into a 5.5 X 3.5 inch rectangular mould and bake in microwave at regular heating mode for 3 minutes.
Once the cake is out of the microwave, place an entire bar of dark chocolate on top of the cake and heat in a microwave at regular settings for about 2 mins, till the chocolate just begins to melt.
Recipe by Bake with Shivesh at https://bakewithshivesh.com/chocolate-bar-cake-in-the-microwave/