01 - Combine balsamic vinegar and honey in a small saucepan. Bring to a gentle boil over medium heat, then reduce to a simmer. Cook for 5 to 7 minutes, stirring occasionally, until reduced by half and syrupy. Remove from heat and let cool.
02 - Lightly mash fresh blackberries in a bowl with a fork, leaving some larger pieces for texture.
03 - Lay out bread slices. Spread softened butter on one side of each slice. On each unbuttered side, arrange Brie cheese in an even layer. Spoon mashed blackberries over two slices and drizzle each with balsamic reduction.
04 - Top with the remaining bread slices, ensuring buttered sides face outward to form sandwiches.
05 - Heat a skillet or griddle over medium heat. Place sandwiches in the pan and cook for 3 to 4 minutes per side, pressing gently with a spatula, until bread is golden-brown and Brie has melted.
06 - Slice sandwiches in half with a bread knife and serve warm.