Medieval sweets weren’t much by our standards. Fruit was the dessert of non-aristocrats; later medieval letters record a father sending his son a box of pears from home to his boarding school in town. Although pies were generally meat dishes, they made fruit tarts. They also stewed fruit and added it to other dishes.
There were two basic problems in the evolution of dessert. One was the sweetening agent, the other was how to find a rising agent to make wheat-based sweets soft enough to eat without effort.
Sugar didn’t become available outside of palaces until African slavery was well established in the Renaissance. (Anti-slavery campaigners in later centuries would boycott sugar in protest.) Until then, a loaf of refined sugar had the same market value as the same size brick of silver, so very few medieval people ever saw or tasted it. Honey was available to those who could afford it, and growing wealth in the middle class allowed them to have sweets at holidays.
Without effective leavening, the steamed/boiled pudding was their go-to dessert. At Christmas, they added imported dried plums and figs from Spain; of course this was also the genesis of holiday fruitcake. The pudding was thickened with eggs, put into a bag, and steamed until it cooked solid and could be sliced.
The first recognizable cookie was literally bis-cuit, in French “cooked twice.” It was a heavily spiced gingerbread that began as ginger porridge, its first cooking. When it was very thick and dry, they poured it into molds and baked it in a bread oven, the second cooking. The final product was still not very sweet, and it was tooth-breaking hard. Since spices were still an upper-class marker, gingerbread caught on among the rich. It had to be dipped and soaked in hot broth to make it edible.
Bakers made sweetened breads at holiday times, of course. Until the late Middle Ages, when some bakers began borrowing brewers’ yeast, all breads were sourdough. When they began producing honey-sweetened yeast bread with imported Spanish raisins, they began to have a food worthy of being called a dessert.
The last option was to boil something in honey solution until it became candied. Whole spices, nuts and roots like horseradish could be candied this way.