In a medium saucepan over medium heat, using a wooden spoon, stir together butter, honey, and salt until butter is melted. Increase heat and bring to a low boil. Boil honey mixture gently for 1 minute, stirring constantly. Stir in vanilla.

Carefully pour honey mixture over popcorn in roasting pan and stir to coat. Bake popcorn, stirring every 5 minutes, until deep golden all over, about 20 minutes.

Let popcorn stand 5 minutes until it is cool enough to handle. Working quickly with lightly buttered hands, press small handfuls of the mixture into 1 ½-in. balls, If mixture starts to harden too quickly while shaping balls, return it to oven for about 45 seconds to soften.

Let popcorn balls cool completely. Drizzle with melted dark chocolate and sprinkle with fleur de sel.

Store in an airtight container at room temperature for up to 2 weeks.