01 - In a small saucepan, combine balsamic vinegar and honey. Bring to a gentle boil over medium heat, then reduce to a simmer. Cook for 5 to 7 minutes, stirring occasionally, until the mixture is reduced by half and has a syrupy consistency. Remove from heat and allow to cool.
02 - Gently mash the fresh blackberries with a fork in a bowl, leaving some larger pieces for texture.
03 - Place the bread slices on a clean surface. Spread the softened butter on one side of each slice. On the unbuttered sides, evenly layer Brie cheese. Top two slices with the mashed blackberries and drizzle with the balsamic reduction.
04 - Close the sandwiches by placing the remaining bread slices on top, buttered side facing outward.
05 - Preheat a skillet or griddle over medium heat. Place the assembled sandwiches on the hot surface and cook for 3 to 4 minutes per side, gently pressing with a spatula, until the bread is crisp and golden and the Brie is melted.
06 - Transfer grilled sandwiches to a cutting board, slice in half with a bread knife, and serve warm.