Sift the popped popcorn, with your fingers, from one bowl to another in order to lose most of the unpopped kernels.
Stir and melt together butter, sugar, and corn syrup in a medium saucepan over medium heat. Once butter is melted, let it come to a boil and cook for 1 minute, without stirring. Remove from heat and stir in vanilla.
Immediately pour over the popcorn. You can also add 1 cup of pecan halves, almonds, and/or ribbon coconut. Stir for a few minutes to coat evenly. Let cool slightly, then serve.