Heat oil and rosemary sprigs in a large Dutch oven over low heat, stirring occasionally, until rosemary begins to brown. Immediately remove and discard rosemary sprigs.
Add popcorn to pot; cover, increase heat to medium, and cook, shaking pot occasionally, until the majority of kernels have popped. Transfer popped corn to a large bowl.
Whisk together melted butter, truffle oil, and rosemary. Drizzle over popped corn; toss. Season with kosher or flaked salt to taste.
