From The Poetical Decameron, Or, Ten Conversations on English Poets and Poetry, Vol. 2, by John Payne Collier (Edinburgh: Archibald Constable and Co., 1820), page 71:

MORTON. First letting us a little more into the secret about that book you call Epulario.

BOURNE. Here it is, at your service, and you will find it nothing more than an old cookery book, affording a little amusement on account of the strangeness of some of the dishes: for instance the following, "To make Pies so that the Birds may be aliue in them and flie out when it is cut vp."

ELLIOT. That is certainly of the utmost value, being, no doubt, the origin of that famous old ballad, the delight alike of babies and bibliographers;

"Sing a song of sixpence, a pocket full of rye,
Four and twenty blackbirds baked in a pie;
When the pie was open'd the birds began to sing,
Was not that a dainty dish to set before the king?"

Read it by all means.

BOURNE. I will, a part of it; not to gratify your love of ridicule, but because it affords a happy note of illustration to Shakespeare's expression, "a custard coffin," in his "Taming of the Shrew." "Make (says the translator of Epulario, for it is from the Italian), the coffin of a great Pie or pasty, in the bottome whereof make a hole as big as your fist, or bigger if you will; let the sides of the coffin be somewhat higher then ordinary Pies, which done put it full of flower and bake it, and being baked open the hole in the bottome and take out the flower."

MORTON. And put the living birds in its place, that, I take it, is the great secret.

BOURNE. You have guessed it exactly, and we need read no more of it.