1. Preheat oven to 350ºF. Spread almonds on an ungreased, rimmed baking sheet; bake until golden and fragrant, 5 to 7 minutes, stirring twice. Transfer to a bowl to cool. Reduce oven temperature to 250ºF; wipe off baking sheet.
2. Spread popcorn and almonds in a single layer on baking sheet. In a pan over medium heat, combine brown sugar, butter, corn syrup and salt. Bring to a boil; let boil 5 minutes. Remove from heat and stir in baking soda and vanilla. Pour brown sugar mixture over popcorn mixture and stir to coat. Bake for 1 hour, stirring every 15 minutes. Let cool, then crush slightly.
3. Line a rimmed baking sheet with foil. Place chocolate in a heatproof bowl set over a pan of simmering water. (Do not let water boil or let bottom of bowl touch water.) When chocolate is melted, pour it onto baking sheet; spread evenly. Top with popcorn mixture, pressing gently to adhere. Sprinkle with sea salt, if desired. Refrigerate until set, about 3 hours. Break bark into pieces and store in an airtight container between sheets of waxed paper.