Jamoncillo is a traditional dessert from Mexico. It is made by cooking milk and sugar, optionally with other flavorings, over low heat until the mixture thickens and caramelizes. The rich, sweet paste is then poured into molds and allowed to set on a flat surface.
The most common version made with only sugar and milk is called jamoncillo de leche. When nuts (usually pecans) are added, it becomes jamoncillo de nuez. It can also be layered in multiple colors and flavors.
Note possible confusion with jamoncillo de pepita, a related sweet that is made with calabash seeds.