Preheat oven to 350°. Cook butter in a medium saucepan over medium heat, stirring constantly, 3 to 5 minutes or just until butter begins to turn golden brown. Immediately remove pan from heat, and stir in pecans. Arrange pecans in a single layer on a baking sheet. Sprinkle with salt and sugar.
Bake pecans at 350° for 10 to 12 minutes or until toasted and fragrant, stirring halfway through. Sprinkle with rosemary. Bake 2 more minutes. Cool completely on baking sheet (about 30 minutes). Store in an airtight container.