In a large bowl, combine popcorn and nuts. In a small heavy saucepan, combine the brown sugar, butter, corn syrup and cream of tartar; stir until sugar is dissolved. Cook, without stirring, over medium heat until a candy thermometer reads 300°-310° (hard-crack stage).
Remove from the heat; quickly stir in baking soda and extract until mixture is light and foamy. Immediately pour over popcorn mixture; toss gently. Spread into two greased 15x10x1-in. baking pans. Press gently to flatten. Cool completely. Break into pieces.