Combine butter and 1/4 cup chocolate chunks in medium microwave-safe container and heat at 30 second intervals until evenly melted, stirring between each. Add vegetable oil and 1 cup granulated sugar and stir thoroughly. Add cocoa powder, 1-1/2 teaspoons vanilla, icing color, vinegar and salt, stirring after each addition. When fully combined, alternately stir in 2 eggs and flour. Fold in remaining 1/2 cup chocolate chunks. Reserve 1/4 cup batter for topping, and pour remaining batter into prepared pan.