01 - In a small saucepan, combine balsamic vinegar and honey. Bring to a gentle boil, reduce heat, and simmer for 5–7 minutes, stirring occasionally, until the mixture reduces by half and becomes syrupy. Remove from heat and allow to cool.
02 - With a fork, lightly mash the blackberries in a bowl, leaving some larger pieces for texture.
03 - Spread butter on one side of each bread slice. On the unbuttered sides, evenly layer the Brie cheese. Spoon mashed blackberries onto two slices, then drizzle each with balsamic reduction. Top each with the remaining bread slices, buttered side facing outward.
04 - Heat a skillet or griddle over medium heat. Place sandwiches in the pan and cook for 3–4 minutes on each side, pressing gently with a spatula, until golden-brown and Brie is melted.
05 - Slice sandwiches in half with a bread knife and serve warm.