Step 2: Combine the Base Ingredients
In a very large mixing bowl — larger than you think you need, because tossing everything together without spilling requires more room than the volume of ingredients suggests — combine the popped popcorn and the mixed nuts. Toss them together briefly to distribute the nuts evenly throughout the popcorn. If any of the popcorn has clumped together, break the clumps apart gently with your hands before adding the coating, so the mixture bakes evenly rather than with dense pockets of stuck-together popcorn in the middle.
Step 3: Make the Coating
In a small saucepan over medium heat, or in the microwave in 30-second intervals, melt the butter completely. Add the brown sugar to the melted butter and stir until fully dissolved and the mixture is glossy and smooth — this takes about one minute of stirring over medium heat, or about two rounds of 30-second microwave intervals with thorough stirring between each. The mixture should look like a thin, amber-colored caramel sauce. If you are adding vanilla extract or spices, stir them in now. Do not allow the mixture to come to a full rolling boil, which can cause the butter to separate from the sugar and produce a grainy rather than smooth coating.
Step 4: Coat Everything
Pour the warm butter-sugar mixture in a thin, even stream over the popcorn and nuts in the large bowl, stirring constantly with a wooden spoon or sturdy spatula as you pour. The goal is to coat every piece of popcorn and every nut with a thin, even layer of the mixture — work quickly but thoroughly, because the coating will begin to thicken and set as it cools, and clumps that form in the bowl will not distribute evenly once the mixture has set on the surface. Once everything is coated, spread the mixture in a single, even layer on the prepared baking sheet. Use the back of your spoon or spatula to press it out into a roughly even layer, though some natural clumping is fine and actually desirable in the finished product.
Step 5: Bake Low and Slow
Place the baking sheet in the preheated oven and bake for 45 minutes to one hour, removing the pan every 15 minutes to stir the mixture gently with a spatula and redistribute it across the pan. This periodic stirring is the step that makes the difference between an evenly coated, uniformly caramelized snack mix and one with burned edges and underdone center pieces. The mixture is ready when it smells deeply caramelized and nutty, has turned a few shades darker than when it went in, and the coating has dried and set enough that individual pieces are no longer sticky to the touch when you press one gently with your fingertip. At 250 degrees, this typically takes about 50 to 60 minutes total, though it can vary based on the moisture content of your popcorn and the humidity in your kitchen.
