Rosemary Caprese Skewers

In a blender, puree the balsamic vinegar, plum preserves, and brown sugar until smooth. Pour into small to medium saucepan and bring to a boil over medium high heat. Turn the heat to simmer and cook until the the mixture has reduced by half. It should be fairly thick and will thicken further as it cools. When it is cool, put it into a squeeze bottle and refrigerate for 1 1/2 to 2 hours (or longer if needed). Marinate mozzarella in the olive oil, red pepper flakes, salt, and pepper. Cover with plastic wrap and refrigerate for at least an hour. Trim the leaves off the bottoms of the rosemary branches. Skewer on one to two mozzarella balls, then a tomato. Place onto a platter, cover, and refrigerate until ready to serve. Squeeze on the plum reduction before serving.