01 - Combine balsamic vinegar and honey in a small saucepan. 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 syrupy. Remove from heat and allow to cool.
02 - Using a fork, lightly mash blackberries in a bowl, leaving several larger pieces for texture.
03 - Lay out bread slices and spread butter on one side of each. On the unbuttered sides, arrange Brie slices evenly. Top two bread slices with the mashed blackberries and drizzle each with balsamic reduction. Cover with remaining bread slices, buttered side out, to close the sandwiches.
04 - Preheat a skillet or griddle to medium heat. Place the assembled sandwiches onto 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 Brie has melted.
05 - Transfer to a cutting board, slice in half, and serve immediately while warm.