Six specimens of William Shakespeare's signature survive. In none of them does he spell his own name "Shakespeare". Three of the signatures abbreviate his name. Of those that do not, two use the spelling "Shakspere" and one "Shakspeare". None of the specimens, abbreviated or full, have an 'e' following the 'k'.
This suggests that the poet's name was most often spelled (or spelt) "Shakspere". Accordingly, a few centuries later, in 1875, Edward Dowden entitled his magisterial work Shakspere: A Critical Study of his Mind and Art. Dowden's other works also use the spelling "Shakspere".
Yet in our day, the spelling "Shakespeare" is ubiquitous. Whence and wherefore this particular orthography, which seems entirely unattested by historical evidence? How and why did the spelling "Shakespeare" become the universally accepted norm for the poet's name?
(Let us set aside as frivolous and/or misguided the attempts to spell that name "Bacon" or "Oxford".)