Preheat oven to 400°F. Lightly grease 12 muffin cups or line with paper baking cups. Set aside.
Mix flour, 2/3 cup sugar, baking powder, baking soda and salt in large bowl. Mix sour cream, milk, oil, egg and vanilla in medium bowl. Add to flour mixture; stir just until dry ingredients are moistened. (Batter will be thick and slightly lumpy.) Gently stir in chocolate chips.
Spoon batter into prepared muffin cups, filling each cup 2/3 full.
Bake 20 to 25 minutes or until toothpick inserted in center of muffins comes out clean. Serve warm.