Preheat oven to 350F. Line 18 muffin pans with cupcake liners. Cream butter and sugar together until fluffly. Add the eggs one at a time and then beat in vanilla, sour cream, milk and grapefruit juice. Mix well. In another bowl whisk the flour, baking powder, soda and salt together. Slowly add the flour into the butter mixture and mix until just incorporated. Stir in the zest. Spoon into cupcake liners ⅔ to ¾ full. Bake for 18-20 min. until they bounce back to a light touch and are lightly golden. Let cool on wire racks completely.