To begin with first to a mixing bowl add flour, baking soda and salt.
Mix it well with a whisk for even mixing, set aside. You can alternatively sieve it too.
To another mixing bowl - add sugar and bournvita.
Powder it fine using a mixie. Use a dry mixer jar. Transfer to the bowl and set aside.
To a big mixing bowl add warm water, oil and vinegar.
Now add powdered sugar bournvita mixture to it.
Mix it well without lumps. Mix until sugar and bournvita dissolves completely.
Add in flour mixture.
Add remaining warm water.
Whisk to form a smooth flowing batter.
The batter should be thick but flowing.
Prepare the cake tin by placing butter paper. Brush with oil/butter, sprinkle wheat flour. Tap to remove excess flour.
Pour the batter inside the cake tin.
Preheat the kadai with steel stand placed inside.
Cook covered for 35-40 mins or until a tooth pick inserted in center comes out clean.
Cake is done id the toothpick inserted in middle comes out clean or with cake crumbs. If baking in oven bake at 180 deg C for 30 - 35 mins or until a toothpick inserted in middle comes out clean. Don't worry if the cake cracks out, its normal.
Let the cake cool down for few mins then invert to demould.
Remove butter paper, let the cake cool down completely.