Put the cereal in a large bowl. Combine the brown sugar, corn syrup, heavy cream and butter in a small saucepan. Bring to a simmer over medium-high heat, then reduce the heat to medium and whisk until smooth. Continue cooking, whisking occasionally, until the mixture thickens and starts to pull away from the sides of the pan, 5 to 6 minutes (it should register 230 degrees F on a candy thermometer). Remove from the heat and whisk in the vanilla, nutmeg and salt.
Immediately pour the caramel over the cereal and stir with a rubber spatula until evenly coated. Scoop heaping tablespoonfuls of the cereal mixture onto an ungreased baking sheet. Let cool slightly, then use your fingers to gently form each into a mound or pyramid shape; sprinkle with nonpareils. Let cool completely.