01 - In a small saucepan, combine balsamic vinegar and honey. Bring to a gentle boil, then reduce heat to a simmer. Cook for 5 to 7 minutes, stirring occasionally, until the mixture is reduced by half and develops a syrupy consistency. Remove from heat and set aside to cool.
02 - Gently mash the blackberries in a bowl using a fork, leaving some pieces intact for texture.
03 - Arrange the bread slices on a work surface. Spread softened butter on one side of each slice. On the unbuttered sides, distribute the Brie cheese evenly. Spoon mashed blackberries onto two of the slices and drizzle each with balsamic reduction. Top with the remaining bread slices, buttered side out.
04 - Preheat a skillet or griddle over medium heat. Place assembled sandwiches in the pan and cook for 3 to 4 minutes per side, pressing gently with a spatula, until the bread is crisp and golden and the cheese is melted.
05 - Transfer sandwiches to a cutting board. Slice in half and serve warm.