Mix butter, brown sugar, corn syrup and salt in a small saucepan. Stir until sugar dissolves, then continue cooking undisturbed until reaches 248°F (firm consistency). Remove from heat and stir in baking soda- mixture will roll vigorously- stir until smooth.
Drizzle sugar mixture evenly over popcorn and nuts. Toss well with a spatula.
Bake 45 minutes, tossing every 10 minutes. Cool on baking sheet; store in covered container.