Preheat oven to 250-degrees F.

Grease baking sheet (with 1-inch sides).

Pop microwave popcorn according to package directions. Put the popcorn in a large mixing bowl. Remove all unpopped kernels. Set the popcorn aside.

Melt butter in saucepan over medium heat. Stir in cake mix, sugar, corn syrup, and salt. Bring to a boil. Boil for 5 minutes, stirring constantly.

Stir in vanilla. Pour over popcorn and stir until popcorn is coated.

Bake at 250°F for 1 hour, stirring every 15 minutes. Remove from oven and immediately top with sprinkles. Cool. Store in airtight container.