01 - Combine balsamic vinegar and honey in a small saucepan. Bring to a gentle boil over medium heat, then simmer for 5 to 7 minutes, stirring occasionally, until the mixture is reduced by half and syrupy. Remove from heat and let cool.
02 - Using a fork, lightly mash the blackberries in a bowl, leaving some larger fragments for texture.
03 - Lay out the bread slices and spread butter on one side of each. On the unbuttered sides, arrange the Brie slices evenly. Top two pieces with the mashed blackberries and drizzle each with the prepared balsamic reduction.
04 - Top the filled slices with the remaining bread, buttered side facing outward, to create two sandwiches.
05 - Heat a skillet or griddle over medium heat. Grill the sandwiches for 3 to 4 minutes per side, pressing gently with a spatula, until the bread is golden-brown and the Brie is thoroughly melted.
06 - Slice sandwiches in half and serve immediately while warm.