01 - In a small saucepan over medium heat, combine balsamic vinegar and honey. Simmer for 3 to 5 minutes, stirring often, until slightly thickened. Remove from heat and allow to cool briefly.
02 - Spread softened butter on one side of each bread slice. Place two slices, buttered side down, onto a clean work surface.
03 - Top each bread slice with an even layer of sliced brie, followed by shredded cheddar and blackberry halves. Drizzle each with approximately 1 teaspoon balsamic glaze.
04 - Place the remaining bread slices over the fillings, buttered side up, to form sandwiches.
05 - Heat a nonstick skillet or griddle over medium heat. Add sandwiches and grill 3 to 4 minutes per side, pressing gently, until golden and cheese is fully melted.
06 - Remove sandwiches from heat, let rest for 1 minute, then slice and serve while warm.