Put the sugar in a suitable container on the stove to warm up and make a light caramel. Carefully add the butter and vanilla, and stir constantly, being careful not to burn the mixture. Pour the popcorn in and stir so that the caramel covers it evenly.
Quickly pour them out into an oiled tray and separate them from one another. Serve after the caramel solidifies.
Another way to do this is to mix the butter and sugar in a saucepan and to add the popcorn after the mixture obtains a caramel color.
Close the lid and wait for the popcorn to start popping, while carefully shaking the pan.