The English word "pastille" (and related words in other languages) obviously derives from the Latin pastillus. I know pastillus had a similar purpose to today's pastille, and, if I remember what I heard years ago, it was something like a small bread seasoned with a lot of herbs. However, I have been unable to find a reliable and detailed description. So, what exactly was the ancient pastillus? Do we have a recipe for making them, for example? Are there present-day food items that match pastillus well?
I can't actually find any reference to pastillus meaning a type of bread.
The OLD defines it as a medical term meaning "a pastille (either swallowed or applied as a medicament)".
I have done a fairly exhaustive (and exhausting - Pliny mentions pastillus 60 times!) word search only to find it always means a pastille or lozenge, typically involving the grinding of powders and liquid or the drying out of liquids which are then formed into pastilli. The only time pastillus is mentioned in conjunction with bread is by Pliny and it refers to the dried cakes of leaven made with wheat bran (Natural History, 18.102)
The other form of the pastillus I encountered is the cake of perfume or breath-freshener: see Horace, Satires, 1.2, line 27 and again at 1.4, line 92; Seneca, Epistles, 86.13; Martial, Epigrams, 1.87.
However, the OLD does have a separate entry for pastillum which is defined as a small sacrificial cake (and perhaps the origins of the Hostia Tom Cotton mentions above?). This has one attestation, Sextus Pompeius Festus, writing in the 2nd century AD.
Perhaps the given etymology of panis + the diminutive suffix -illus has given the impression that it once/also meant bread. It is interesting to note, therefore, that the OED points out that some early Romance-language etymologists claimed its origins were linked to paste/pasta/pâte. Although the OED dismisses this etymology, I find it not so far-fetched given that almost every single mention of pastillus, especially in Pliny, involves making a paste!
PS: In answer to your question about recipes, Pliny has plenty! Although some are a little bizarre!
What you remember from years ago is probably as good a description as you will find.
Dictionaries usually agree that a pastillus was a small, round object baked from flour, which some describe as a "roll". Two classical references commonly appear: Horace, Satires 1. 2. 27, pastillos Rufus olet, and Martial, 1. 88 (which is not quoted literally in any of my sources, and should actually be 1.87 — so much for the independence of lexicographers!), Ne gravis hesterno fragres, Fescennia, vino / pastillos Cosmi luxuriosa voras.
Although I'm unable to confirm it, I once had an older, Catholic acquaintance who believed that pastillum was the Latin word for the unconsecrated disc before it was used as the Hostia in the Mass. That's the closest I can get to a recipe or a present-day food item. It would agree with suggestions of shape and the use (with herbal content) as a breath-sweetener: the word is sometimes translated to English as lozenge, which certainly has a medicinal connotation.
Ainsworth gives three translations for pastillus, each is a small lump made from various ingredients bound with gum/wax/resin and left to harden. All three types would fit the famous quote about Rufus giving off an aroma of pastillus.
Troches are bound with liquorice, a plant native to S. Europe, with some medical qualities, They were made up with a very wide variety of medications including scented herbs (mint and thyme). This example includes water hemlock.
Cubebs, Conium, ***ccia, Liquorice, Sugar (letters lost in the photo)
Pastilles or lozenges are bound with pectin or gelatin, most contain fruit. Violet scented pastilles were used as breath fresheeners, 18th 19th Centuries, and perhaps antiquity.
Pomander is usually bound with ambergris. It is mentioned in medical books because disease was believed to be carried on effluvium. The pomander is carried, not eaten. Warmth from your hand, or the heat of a crowded room would bring out the scent.
Boyle Family Recipes
To Make Excellent Pomander
Take fifteen grains of Musk, thirty grains of ambergreece, five and forty grains of Moss powder, and nine grains of civet. Grind all these in a stone mortar* with some gum Dragon, which must first be steeped in Rosewater with a little civet two days, then grind all together to a perfect paste that it works smooth, then mingle Oyntment of orange flowers with a little civet & Musk to anoint Yo^ hands or Molds that the paste stick not to them. then dry them abroad. Another ...
The translations were found in James Ainsworth Classical section. The recipes were found by using the search box on the Wellcome Library website. If you wish to pursue this seriously, I suggest you try them; they may be able to close the gap between Pliny and the Georgian Age of Reason.
Liber de coquina describes the making of a pastillus by forming a small, circular dough, placing chaff on top, sealing this with more dough, baking under a terra cotta cover, then making a small hole on the bottom of the roll so formed to extract the chaff, thus forming a cavity into which various savory bits could be inserted. A general meaning might be something like 'roll'.
The specific recipe mentioned has small, live, wild birds placed in the cavity and the hole sealed with dough so that when the dish is served to the lords, they will break into the pastry and receive a real surprise! The original 'four and twenty blackbirds'?