Preheat your oven to 170°C/ 325F/ gas mark 3. Line a baking tray with a baking sheet.
Pour the oil over the bottom of a large saucepan that has a lid, and add the popcorn kernels, shimmying the pan around so the kernels land in one layer.
Cover the pot, heat it over a medium to high heat, and, once the kernels begin to pop, keep the saucepan moving until all of the kernels have popped, about 5 to 7 minutes total.
Toss the table salt and then the melted butter over the popcorn, then transfer it to a bowl so that you can fish out any unpopped kernels. Let cool.
In a large bowl, cream together the softened butter, brown sugar, granulated sugar, egg, and vanilla until smooth.
In a sererate bowl, whisk the flour and baking soda together.
Stir the combined dry ingredients into the butter-sugar mixture.
Fold in the cooled popcorn so that it is evenly distributed through the batter, which will seem like a ridiculous instruction because there is so much popcorn and so little cookie batter, but it works. Don't worry if the popcorn breaks up a bit. The mixed-size pieces are part of the cookies charm.
Scoop heaping-tablespoon-sized mounds 5cm apart onto a parchment-lined baking sheet.
Bake the cookies in the oven for 10 to 12 minutes, until the edges are light brown.
Let them sit on the hot baking sheet for a few minutes to firm up before transferring them to a rack to cool.