Step 1Heat the oven to 200 degrees. In a large heat-proof bowl, combine the popcorn, peanuts, almonds and pecans. Place the bowl in the oven to warm while making the caramel. Lightly butter 2 baking sheets.
Step 2In a large heavy-bottomed saucepan, combine the butter, brown sugar, corn syrup and salt. Heat the pan over high heat, stirring to combine the contents, until the mixture comes to a boil. Reduce the heat to a strong simmer and continue cooking, stirring frequently, until a candy thermometer inserted reads 255 degrees. Stir in the baking soda and remove from heat.
Step 3Pour the caramel quickly over the popcorn and toss to coat evenly. Divide the caramel corn between the two baking sheets and place them in the oven. Bake, stirring occasionally, until the caramel dries and hardens, about 1 hour. Remove the trays from the oven and allow the caramel corn to cool on the pans. Break the cooled caramel corn into small clusters and store in airtight containers. The caramel corn will keep for 2 days, stored at room temperature.
Spicy variation: For caramel corn with pine nuts and piment d’espelette (ground Basque chile, available at gourmet stores), follow the basic recipe at left, combining the brown sugar, corn syrup, salt, baking soda and 1 1/2 teaspoons piment d’espelette in a heavy-bottomed saucepan. When tossing the caramel with the popped corn, substitute 2 cups toasted pine nuts for the peanuts, almonds and pecans before placing in the oven. Serve each portion of caramel corn with a lemon wedge, and squeeze a few drops of lemon on the caramel corn just before eating.