01 - Combine balsamic vinegar and honey in a small saucepan over medium heat. Bring mixture to a gentle boil, then reduce heat and simmer, stirring occasionally, until reduced by half and syrupy, about 5 to 7 minutes. Remove from heat and allow to cool.
02 - Gently mash blackberries in a bowl using a fork, leaving some larger pieces for added texture.
03 - Spread a thin layer of softened butter onto one side of each bread slice. Arrange sliced Brie evenly on the unbuttered sides. Layer mashed blackberries on two bread slices and drizzle each with a portion of the balsamic reduction.
04 - Top with remaining bread slices, keeping the buttered sides facing outward to prepare for toasting.
05 - Heat a skillet or griddle over medium heat. Place assembled sandwiches onto the hot surface and cook for 3 to 4 minutes per side, pressing gently with a spatula, until the bread is crisp and golden and the Brie is melted.
06 - Slice each sandwich in half and serve promptly while warm.