Beat together butter, lemon juice, milk, lemon zest and egg yolks until homogeneous (it won't be "smooth" because of the zest). Stir into dry ingredients just until blended.
In a clean bowl, whip egg whites and cream of tartar (by hand or with electric beater) until stiff and glossy. Stir 1/4 of egg whites into base. Gently fold remaining whites into base, taking care not to deflate egg whites too much. Transfer to souffle dish or small casserole. Bake 35 minutes or until almost set (center of souffle should be slightly wobbly when pan is gently shaken). Serve hot or cold.