Heat the popcorn and oil in a 1-gallon, heavy bottomed pot over medium-high heat. When the popcorns starts to pop, cover the pot tightly and continue to cook until all the popcorn pops, about 2 to 3 minutes, shaking the pot occasionally. Pour the popcorn into a large bowl.
In a medium saute pan over medium-high heat, melt the butter and cook until it is brown, shaking the pan occasionally. Remove the pan from the heat. Add the rosemary to the butter and pour the butter mixture over the popcorn. Toss well. Add the salt and pepper and lemon zest. Toss well.