Marble Cake:

Ingredients :

All purpose flour     --   1½ cup
Baking Powder        --   2 tsp
Salt                                --   ¼ tsp
Butter                           --   1½ cup
Granulated Sugar   --   ¾ cup
Large Eggs                 --   2
Vanilla extract         --   1 tsp
Sour cream or plain yogurt --   ¼ cup
Milk                               --   ½ cup
Chocolate                   --   4 oz / 115 gm

Preparation of the cake:

Preheat the oven to 325 degrees F . Butter and lightly flour a 9 x 5 x 3 inch loaf pan. Line the bottom of the pan with parchment paper.
Melt the chocolate, along with ¼ cup of the milk. You can either double boil(safe) or can directly do it in a sauce pan or microwave. Make sure you do not burn the chocolate while melting directly. Remove from heat and keep aside.
In a separate bowl, whisk the flour with the baking powder and salt.
In a small bowl whisk the sour cream with the remaining milk, and the vanilla extract.
Beat the butter until smooth. Add sugar and beat until light and fluffy. Beat in the eggs, one at a time, until well combined.
With the mixer on low speed, alternately add the flour mixture and sour cream/milk mixture, beginning and ending with the flour.
Divide the batter into two.  Add the melted chocolate into one half of the batter and fold in until combined.

With the help of 2 spoons, scoop the batter alternately into the prepared loaf pan. I did my first layer with vanilla - chocolate - vanilla and the second layer with chocolate - vanilla - chocolate and the third layer again with vanilla - chocolate - vanilla. Run a knife or skewer through the batters to get a marbled effect.
Bake in preheated oven for about 50 - 55 minutes or until a toothpick inserted in the center comes out clean.
Remove from oven and place on a wire rack to cool for 10 minutes before removing from pan. Cool completely.

Recipe Courtesy: Joy of Baking