PREHEAT oven to 350º F. Grease and flour 13 x 9-inch baking pan.
MELT 3/4 cup chopped baking bar in small, uncovered, microwave-safe bowl on MEDIUM-HIGH (70%) power for 45 seconds; STIR. Pieces may retain some of their original shape. If necessary, microwave at additional 10- to 15-second intervals, stirring just until smooth. Cool to room temperature.
MICROWAVE 1/4 cup milk and coffee granules in small, uncovered, microwave-safe bowl on HIGH (100%) power for 15 seconds. Stir mixture with whisk until coffee granules are dissolved. Cool to room temperature.
COMBINE nuts, remaining chocolate, 1/3 cup granulated sugar and 2 tablespoons butter in medium bowl until crumbly.
COMBINE flour, baking soda, salt and baking powder in medium bowl. Beat remaining 2 cups granulated sugar and remaining 1/3 cup butter in large mixer bowl until well blended. Beat in sour cream, eggs, remaining 3/4 cup plus 2 tablespoons milk and vanilla extract until blended. Gradually beat in flour mixture. Remove 3 cups batter to medium bowl. Stir in melted chocolate and coffee mixture. Pour chocolate batter into prepared pan.
DROP tablespoons of remaining batter randomly over chocolate batter. Swirl mixture with knife for marbled effect. Sprinkle nut mixture over top.
BAKE for 38 to 40 minutes or until wooden pick inserted in center comes out clean. Cool completely in pan on wire rack.