Step 1

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

Step 2

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

Step 3

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).)

Step 4

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.

Step 5

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).

Step 6

Bake for 1 hour, gently stirring every 20 minutes.

Step 7

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.

Step 8

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.