The Vanishing Viscount by Jane Charles

Lady Clara Westcott fell in love with Pierce Drakos, Viscount Harcourt, at the age of fifteen. That is also when everything went wrong- at least magically. When she encounters him again in London, Clara quickly realizes that she might very well be a danger to Harcourt and vows to keep her distance.
Pierce had considered Lady Clara sweet and pretty at fifteen but by eighteen, she had matured in a most delectable manner. However, every time he wished to draw near, she disappeared. Now that she would be visiting his family’s estate and attending his mother’s Witches’ Ball, he vowed to learn why she avoided him. What he hadn’t counted on was him being the one who now vanished.