PREHEAT oven to 350° F. Grease 13 x 9-inch baking pan.
BEAT flour, sugar, sour cream, milk, butter, eggs, baking soda, salt, vanilla extract and baking powder in medium mixer bowl until well mixed. Reserve 1/2 cup batter. Spread remaining batter into prepared baking pan.
COMBINE reserved batter and hot fudge topping in small bowl. Drop mixture by heaping teaspoon evenly over batter. Pull knife through batter to marble.
BAKE for 40 to 50 minutes or until wooden pick inserted in center comes out clean and top springs back when touched lightly in center. Cool completely in pan on wire rack.
MICROWAVE morsels in medium, uncovered, microwave-safe bowl on MEDIUM-HIGH (70%) power for 2 minutes; STIR. The morsels may retain some of their original shape. If necessary, microwave at additional 10 to 15-second intervals, stirring just until morsels are melted.
BEAT melted chocolate, sour cream and vanilla extract in small mixer bowl until smooth and creamy. Frost cake.