Warm butter in a small saucepan over medium heat. The butter will melt, then foam and bubble, then start to brown. You will notice little brown bits forming at the bottom of the pan. Stir frequently during this process and keep your attention on the pot. The butter can go from wonderfully brown to burnt in just a few moments. You will know the butter is done when it gives off a nutty aroma and turns a golden brown (about 5 minutes). Stir in the rosemary and lemon zest, then pour over hot popcorn while tossing. Season to taste with kosher salt and freshly ground black pepper.