Place 1 bag popcorn in the microwave; cook on high until popping begins to slow down, about 2 1/2 minutes. Repeat with remaining bags. Open bags carefully; divide popcorn between 2 shallow baking dishes. Mix in peanuts.
Preheat oven to 250 degrees F (120 degrees C).
Combine brown sugar, margarine, corn syrup, and salt in a saucepan over medium heat. Bring to a boil, stirring enough to blend. Simmer, stirring constantly, for 5 minutes. Remove from heat; mix in vanilla extract and baking soda until mixture is light and foamy.
Pour brown sugar mixture over the popcorn-peanut mixture; stir to coat.
Bake in the preheated oven, stirring every 25 minutes, until a cooled piece of popcorn is crunchy, about 1 hour.
Line a flat work surface with waxed paper. Pour popcorn onto the waxed paper. Break up any large clusters.
Melt chocolate almond bark in a microwave-safe glass or ceramic bowl in 15-second intervals, stirring after each melting, 1 to 3 minutes. Pour into a resealable plastic bag; snip off 1 corner. Drizzle melted chocolate over popcorn. Let popcorn cool completely.