Preheat the oven to 350°F or 180°C .
Grease and dust a loaf pan with little flour. You can line the pan with a parchment paper also.
Beat together butter, sugar and vanilla essence, till the mixture is light and fluffy.
Add eggs one by one. Don't overbeat the mixture, it may curdle.
Fold in the flour and mix well with light hands.. Add some milk about 2 tbsp at a time, if you think the mixture is thick to adjust the consistency.
Divide the mixture into two parts. One part can be little more than the other as we will be adding cocoa powder to the other. It depends on you how to divide and which mixture you want to keep more.
Mix cocoa powder and 2 Tbsp of sugar with 2-3 tbsp of milk and fold the mix nicely to one of the parts of batter.
Transfer the batters in the pan with alternate layers.
Run a knife or skewer through the batter for swirls.
Bake in the oven for 45-50 minutes. Check the doneness of the cake by a toothpick/skewer test. Insert a toothpick/skewer in the middle of the cake, if it comes out with crumbs, the cake is done, you can take it out.
After 5 mins.take out carefully from the pan and cool on the wire.
Slice and enjoy with your favorite cuppa of coffee or cake or just like that.