Preheat oven to 250°F (120°C). Line a large baking sheet with parchment paper or a silicone baking mat. Set aside.

In a large saucepan, melt the butter over medium heat. Add the brown sugar, corn syrup, and salt. Stir constantly until the sugar is dissolved and the mixture comes to a boil.

Boil for 4-5 minutes, stirring occasionally, until the caramel reaches a soft ball stage (235-240°F / 112-116°C). Remove from heat.

Stir in the vanilla extract and baking soda. The mixture will foam up. Immediately pour the caramel over the popped popcorn in a very large bowl, stirring gently to coat evenly.

Spread the caramel-coated popcorn in a single layer on the prepared baking sheet.

Bake for 30 minutes, stirring every 10 minutes to prevent sticking and ensure even baking. Remove from oven and let cool completely on the baking sheet.

Once completely cool, break the caramel corn into pieces. In a microwave-safe bowl, melt the white chocolate chips in 30-second intervals, stirring until smooth.

Drizzle the melted white chocolate over the caramel corn. Immediately sprinkle with the dry vanilla cake mix and rainbow sprinkles.

Toss gently to distribute the toppings. Let the chocolate set completely before serving, which can be sped up by refrigerating for 10-15 minutes.