Preheat the oven to 200°F (93°C). Line 1 or 2 12×17-inch baking sheets with silicone baking mats or parchment paper.

Pour the popcorn into the largest bowl you have, or spread it onto the prepared baking sheets. (Either way is fine.)

Combine the sugar, corn syrup, butter, salt, and cream of tartar in a medium saucepan over medium heat. Stir constantly as the butter melts and mixture comes to a simmer. Once simmering around the edges, allow to bubble, without stirring, for 4 minutes. (If you have a candy thermometer, the caramel should reach 234–235°F (112–113°C).)

Remove caramel from heat and immediately stir in the baking soda. The mixture will bubble up, almost double in volume, and appear frothy on the surface as you stir; this is normal and expected. Be careful, it’s very hot.

Pour the caramel over the popcorn and, with a large spoon or spatula, stir gently until all of the popcorn has been coated. Go slowly here and have patience; it requires quite a few turns of the spatula/spoon as the caramel begins to harden. If it’s not already, spread the popcorn evenly onto prepared baking sheet(s).

Bake for 1 hour, gently stirring every 20 minutes.

Allow to cool on the pan for at least 30 minutes. The caramel hardens as it cools. Once cool enough to handle, you can break apart any large clusters.

Cover the popcorn tightly once cooled. This caramel corn stays fresh for up to 2 weeks in an airtight container, so it’s a great recipe to make ahead of time.