01 - In a small saucepan, combine balsamic vinegar and honey. Bring to a gentle boil, then reduce heat and simmer for 5 to 7 minutes, stirring occasionally, until mixture is reduced by half and becomes syrupy. Remove from heat and allow to cool.
02 - In a bowl, lightly mash blackberries with a fork, leaving larger pieces to retain texture.
03 - Lay all bread slices on a flat surface. Spread softened butter on one side of each slice. On the unbuttered sides, arrange sliced Brie evenly across all slices.
04 - Top two Brie-covered slices with the mashed blackberries. Drizzle each with balsamic reduction for concentrated flavor.
05 - Place the remaining bread slices on top of the assembled layers, buttered side facing outward, to complete each sandwich.
06 - Preheat a skillet or griddle over medium heat. Cook sandwiches for 3 to 4 minutes per side, pressing gently with a spatula, until bread is golden-brown and Brie has melted.
07 - Transfer to a cutting board, slice sandwiches in half with a bread knife, and serve while warm.