01 - Combine balsamic vinegar and honey in a small saucepan. Bring to a gentle boil over medium heat, then reduce to a simmer. Stir occasionally and cook 5 to 7 minutes, until the mixture has reduced by half and is syrupy. Remove from heat and allow to cool.
02 - Lightly mash blackberries with a fork in a small bowl. Leave some larger pieces intact to maintain texture.
03 - Arrange bread slices on a work surface. Spread softened butter on one side of each slice. Flip two slices so the unbuttered side faces up and layer evenly with sliced Brie.
04 - Top the Brie-covered slices with mashed blackberries. Drizzle each with prepared balsamic reduction.
05 - Place remaining bread slices on top, buttered side out, to form sandwiches.
06 - Preheat a skillet or griddle over medium heat. Add sandwiches and cook for 3 to 4 minutes per side, pressing gently with a spatula, until bread is golden brown and Brie is melted.
07 - Transfer sandwiches to a cutting board, slice in half, and serve immediately while warm.