In a blender, whirl 1/4 cup plain whole yogurt, 1 clove minced garlic, 2/3 cup packed fresh mint leaves, 1 tsp. salt, 1 tsp. lemon juice, 1/4 tsp. freshly ground black pepper, and 1/8 tsp. cayenne until smooth. Transfer to a serving bowl and stir in an additional 3/4 cup plain whole yogurt.