01 - In a small saucepan, combine balsamic vinegar and honey. Bring the mixture to a gentle boil over medium heat, then reduce to a simmer. Cook for 5 to 7 minutes, stirring occasionally, until the mixture has reduced by half and has a syrupy consistency. Remove from heat and set aside to cool.
02 - Place blackberries in a bowl and gently mash with a fork, leaving some pieces intact for texture.
03 - Lay out bread slices and spread softened butter on one side of each slice. On the unbuttered sides, arrange sliced Brie evenly, then top two slices with mashed blackberries. Drizzle each with prepared balsamic reduction.
04 - Position the remaining bread slices on top, buttered sides facing outward, to fully assemble each sandwich.
05 - Heat a skillet or griddle over medium heat. Place sandwiches on the hot surface and cook for 3 to 4 minutes per side, pressing gently with a spatula, until the bread is golden and the Brie has melted.
06 - Transfer grilled sandwiches to a cutting board, slice in half, and serve immediately while warm.