Notwithstanding any of the foregoing provisions of this part, where, before the expiration of the period prescribed therein for the assessment of a tax, a taxpayer has consented, in writing, that such period be extended, the amount of tax due may be assessed at any time within such extended period. The period so extended may be extended further by subsequent consents, in writing, made before the expiration of the extended period.
72 P.S. § 7261