Place the popped popcorn in a large bowl, discarding any unpopped kernels.

Preheat oven to 300° F. Line a cookie sheet with parchment paper and set aside.

In a 1–2-quart saucepan, combine butter and corn syrup over medium heat, stirring occasionally until butter is melted. Add sugar and stir to combine. Continue to stir mixture until it comes to a full boil. Turn heat to medium low and cook without stirring for 2 minutes.

Remove pan from heat. When boiling action subsides, stir in the red food coloring, then the watermelon flavor.

Immediately pour the candy mixture over the popcorn and mix well.

Transfer coated popcorn to cookie sheet and spread into a single layer.

Bake in preheated oven for 20 minutes, stirring after 10 minutes.

Remove popcorn from oven and allow to cool completely.

Once popcorn is cool, break into pieces and mix in the mini chocolate chips, if desired.