Sauté onion and garlic in butter until tender. Add carrots and 1 cup of the chicken broth. Cover and cook 10 minutes until carrots are tender. Transfer mixture to blender or food processor; process until smooth. Return to pan. Dissolve cornstarch in remaining 2 cups broth; add to carrot mixture with milk and pepper. Cook, stirring until soup just begins to boil and is slightly thickened. Whisk in yogurt. Serve immediately.