Puree balsamic vinegar, plum preserves, and brown sugar in a blender until smooth.

Pour mixture into a saucepan and bring to a boil over medium-high heat.

Reduce heat to simmer and cook until the mixture has reduced by half, becoming fairly thick.

Allow the reduction to cool; it will thicken further.

Transfer the cooled reduction to a squeeze bottle and refrigerate for at least 1.5 hours.

Marinate mozzarella balls in olive oil, red pepper flakes, salt, and pepper.

Cover and refrigerate the mozzarella for at least one hour.

Trim leaves off the bottoms of rosemary branches.

Skewer 1-2 mozzarella balls and a grape tomato onto each rosemary twig.

Arrange skewers on a platter, cover, and refrigerate until serving.

Drizzle the plum reduction over the skewers just before serving.