Line a sheet pan with parchment paper or foil and grease with nonstick cooking spray. I prefer parchment.
In a medium saucepan, combine the water, corn syrup, granulated sugar. Heat the mixture to 300 ºF (149 ºC), which is the hard crack stage.
Add the butter, and continue heat to 330–340ºF (166–171 ºC). This is past the hard crack stage and your mixture will have a light caramel colour to it. Remove the pan from the heat and immediately stir in the popcorn and the salt.
Working quickly, transfer the mixture to the prepared sheet, top with another layer of parchment and roll to the desired thickness. At this point, you can grease a chef's knife and score the brittle into small pieces to make cutting easier. When it's cold, snap it into pieces and store in a container.