Wash the basil and shake dry. Pluck the leaves. Peel and chop the garlic. Squeeze lime. Cut avocado in half, remove stone. Remove the flesh of the avocado from the skin with a spoon.
Puree the avocado pulp with lime juice, basil, garlic, yoghurt and milk in a tall mixing bowl using a hand blender. Season the avocado dressing with salt, pepper and sugar.