1. Heat oven to 350°F. Grease and flour 13x9x2-inch baking pan.
2. Stir together flour, sugar, cocoa, baking soda and salt in large bowl. Add butter, sour cream, eggs and vanilla; beat on medium speed of mixer 3 minutes. Pour batter into prepared pan.
3. Bake 40 to 45 minutes or until wooden pick inserted in center comes out clean. Cool in pan on wire rack. Frost with FUDGE FROSTING. 12 to 15 servings.
3 tablespoons butter or margarine
1/3 cup HERSHEY'S Cocoa
1-1/3 cups powdered sugar
2 to 3 tablespoons milk
1/2 teaspoon vanilla extract
Melt butter in small saucepan over low heat. Add cocoa; cook, stirring constantly, just until mixture begins to boil. Pour mixture into medium bowl; cool completely. Add powdered sugar alternately with milk to cocoa mixture, beating to spreading consistency. Stir in vanilla. About 1 cup frosting.