01 - In a small saucepan, combine balsamic vinegar and honey. Bring to a gentle boil over medium heat, then reduce the heat and simmer 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 - Place the blackberries in a bowl and lightly mash with a fork, leaving some larger pieces intact for added texture.
03 - Lay out the bread slices. Spread butter on one side of each slice. On the unbuttered sides, layer Brie cheese evenly. Top two slices with the mashed blackberries and drizzle each with the balsamic reduction.
04 - Cover with the remaining bread slices, keeping the buttered sides facing outward to enclose the filling.
05 - Heat a skillet or griddle over medium heat. Place the assembled sandwiches on the skillet and cook for 3 to 4 minutes per side, pressing gently with a spatula, until the bread is golden brown and the Brie is fully melted.
06 - Slice the sandwiches in half and present immediately while warm.