Preheat oven at 180 degree celsius.

Beat egg, vanilla essence and sugar until fluffy and add butter to it.

In another bowl combine flour, salt, baking powder and add the egg mixture in to it and fold it well. Add milk to it and fold until the mixture falls in a ribbon manner.

Keep 3/4 cup of the batter aside and add cocoa powder to it and fold it well.

Butter the baking pan and add a layer of the white cake batter, spread it evenly and add the cocoa cake batter as the next layer and finally top it with the rest of the white cake batter. insert a fork/knife and make swirls in the batter.

Bake for 30-35 mins, until a toothpick comes out clean.