I am keener on this dish than Alys is. I like it because it was a genuinely popular and long-lived medieval dessert. A leche seems to be something moulded that you slice to eat.
Forme of Cury has a version of this with pork meat in it, as a kind of meatloaf shaped into a giant pea pod. But that’s an outlier, and I haven’t attempted it.
Usually (eg Harleian MS 279) it is made from dates simmered in wine till soft, and sugar, then add some powdered ginger and cinnamon.
I use the regular dried dates, because I figure that’s what they were like by the time they got to England, and it’s cheaper for a feast. The wine is cheap cask white wine. A 500g packet of dates needs 1L of wine, 500g of breadcrumbs, 100g sugar, 2 tsp ginger, 2 tsp cinnamon.
The Harleian recipe says you can thicken with cooked eggyolks or grated bread (read: breadcrumbs), till you get it quite thick. Frankly, it’s a bit too rich and dark without the breadcrumbs.
Classically, you roll the mixture into a sausage, then cut it into rounds.
It does come out of moulds well, so that’s an option too. These days you can easily get fabulous moulds online for almost anything imaginable. Give them a little spray oil first, push in the leche, and refrigerate. Hopefully the inserts will peel off easily. Practice well before the event! Doesn’t work too well on the finer moulds because the date mixture is a bit speckledy so you can’t see the detail.
(1) in small rounds, (2) as an acorn mould, with carrot suckets.

