Preheat oven to 325. Grease and lightly flour loaf pan.
Melt chocolate and 1/4 cup milk over double boiler. When melted, remove from heat and set aside.
In a bowl, whisk flour with baking powder and salt.
In a small bowl whisk sour cream, remaining milk and vanilla.
With electric mixer, beat butter and sugar until fluffly, add eggs one at a time until well combined. Continue using mixer on low speed and alternately add flour mixture then sour cream mixture. Make sure you begin and end with flour mixture.
Spoon half the batter into a bowl. Add chocolate mixture to one half of batter and carefully fold in.
Make sure you use 2 spoons to add batter to pan. Spoon in white batter, then chocolate batter. Alternating until all batter is in loaf pan. Gentley swirl batter with wooden skewer or knife. Make sure you don't over mix or you won't have the swirl effect!
Bake for 50-55 min. or until toothpick comes out clean. Cool for about 10 mins. before removing from pan.