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.
