Step 1 Heat 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 2 In 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.