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1
Place popcorn in a large shallow roasting pan in one layer, or use two 9-by-13-inch pans; set aside.
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2
Melt the butter in a large heavy saucepan over low heat. Stir in the brown sugar, salt, and corn syrup. Bring to a boil, stirring constantly. Insert a candy thermometer into pan, without the tip touching the bottom of pan. Boil mixture slowly, without stirring, until the temperature reaches the soft crack stage, about 285 degrees. This could take at least 45 minutes, but watch thermometer temperature carefully. Meanwhile, pre-heat oven to 250 degrees.
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3
When the temperature of the mixture is reached, immediately remove from heat and stir in the soda and vanilla. This will cause the mixture to briefly foam up.
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4
Stir the mixture and then immediately pour evenly over the popped corn in the roasting pan. Using 2 wooden spoons, quickly mix to coat all the popcorn; spread evenly on pan.
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5
Bake at 250 degrees for 15 minutes; stir well. Return to oven for another 15 minutes.
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6
Remove pan from oven and stir in walnuts and pumpkin seeds and sprinkle with pumpkin pie spice, coating well. Bake for 30 minutes more, stirring every 10 minutes.
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7
Turn caramel corn out onto non-stick foil or parchment-lined baking sheets in a single layer. Cool completely, then break apart into desired sized pieces. Keep stored in airtight containers.
