Preheat oven to 300 degrees F. Butter (or spray with cooking spray) a large rimmed cookie sheet. Set aside.
Pop popcorn and measure out into a large bowl. Add cashews and toss to combine.
In a small saucepan, bring brown sugar, butter, salt and water to a boil, stirring constantly. Drizzle over popcorn and quickly stir to combine.
Spread popcorn evenly on prepared cookie sheet. Bake, tossing occasionally, until golden brown, about 40 minutes. Transfer onto wax paper to let cool. Enjoy!