Variances: to hear and grant applications for variances as will not be contrary to the public interest, where, owing to special conditions, a literal enforcement will result in practical difficulty or unnecessary hardship, so that the spirit and purposes of this chapter shall be observed and the public safety, welfare, and justice secured. Use variances shall not be granted. In every case where a variance from these regulations has been granted by the Board of Appeals, the minutes of the Board shall affirmatively show that a practical difficulty or unnecessary hardship exists and the records of the Board shall clearly show in what particular and specific respects a practical difficulty or an unnecessary hardship is created.