Preheat the oven to 250°F.

Prepare 5 quarts of popped popcorn. Remove and discard any unpopped kernels. Transfer the popped popcorn to a large turkey roaster or very large roasting pan.

Evenly sprinkle 2 cups shelled peanuts over the top of the popcorn, trying to keep most peanuts on top rather than letting them fall to the bottom.

In a large saucepan, melt 4 ounces butter over medium heat.

Add 1 cup brown sugar, 1/4 cup corn syrup (light or dark), and 1/8 teaspoon salt to the melted butter. Stir constantly until the sugar dissolves and the mixture comes to a full boil.

Once boiling, let the syrup boil, without stirring, for 2 minutes. Time this carefully to avoid burning.

Remove the pan from the heat. Stir in 1 teaspoon vanilla.

Immediately add 1 teaspoon baking soda and stir vigorously. The mixture will foam—be careful to keep your face and hands away from the foam to avoid burns.

While the syrup is foaming, pour it evenly over the popcorn and peanuts in the roaster. Use a large wooden spoon (or heatproof spatula) to gently but thoroughly mix so the popcorn and peanuts are evenly coated.

Place the roaster in the preheated oven. Bake for up to 1 hour and 15 minutes, stirring and turning the popcorn every 15 minutes to separate clusters and promote even coating and drying.

When the popcorn feels dry and crisp (after repeated 15-minute stirs during the baking time), remove the roaster from the oven.

Carefully spread or press the hot popcorn onto a clean, heat-resistant surface to help it cool more quickly, or leave it in the roaster to cool. Let it cool about 30 minutes or until completely cool and set.

Break the cooled popcorn into clumps or pieces and store in an airtight container. Note: humidity will soften the popcorn; if it becomes soggy, return it to a 250°F oven and repeat the stirring/baking process until it is crisp again.

Take care when working with hot syrup—its temperature can cause serious burns.