Preheat oven to 170 C. Grease and flour a 9? round baking pan.
Sieve maida, baking pwd and salt in a bowl.
In another bowl, combine butter and sugar. Use a hand blender to make a smooth creamy mixture.
Add one egg at a time, beating each egg for 1 mt. Add vanilla essence and mix.
Using low speed, mix in the flour mixture little by little alternating with milk. Do not overbeat, just mix till flour is combined.
Remove a cup of this batter in a bowl and combine cocoa pwd. Mix it and do not over beat.
Pour a tbsp of white colored batter into the greased and floured pan. Alternate with a tbsp of cocoa batter. Continue through out till both the batters are finished.
Take a knife and swirl it around the batter in a zig zag fashion, 3-4 times.
Bake in preheated oven for 45 mts or till a toothpick inserted into the cake comes out clean. If the cake is browning too quickly on the top, remove from oven, cover with an aluminum foil and bake till done. Place on a wire rack to cool for 15 mts. Slice and serve at room temperature.