So, three things:
Number One: I'm posting this one mostly because I like the name.
Number Two: Strictly speaking, this is more of a Renaissance England than a Medieval recipe - or at least, our best records on syllabub "only" go back about five hundred years.
Number Three: You can find dozens of recipes for syllabubs. This is just one.
Syllabubs were a kind of custardy, frothy desert, served cold (or cool - this is pre-refrigeration). They were also a combination of ingredients that not too many modern cooks might think of...so if you actually try to make it, let me know how it tastes!
One Quart of Cream
One Pint and an half of Wine or Sack
Juice of two lemons with some of the peel
A branch of rosemary.
Sweeten it very well, then put a little of this liquor, and a little of the cream into a basin, beat them till it froths, put that froth into the syllabub pot, and so do till the cream and wine be done, then cover it close, and set it in a cool cellar for twelve hours, then eat it.(from Hannah Wooley, The Queen-Like Closet, London 1649)