01 - In a small saucepan over medium heat, combine balsamic vinegar and honey. Simmer gently for 3 to 5 minutes until the mixture thickens. Remove from heat and let cool slightly.
02 - Spread softened butter on one side of each bread slice. Arrange two slices, buttered side down, on a clean work surface.
03 - Evenly layer brie slices over the bread, then scatter shredded cheddar and halved blackberries on top. Drizzle each open-faced sandwich with 1 teaspoon of balsamic glaze.
04 - Cover the fillings with the remaining bread slices, buttered side facing up.
05 - Heat a nonstick skillet or griddle over medium heat. Cook sandwiches for 3 to 4 minutes per side, pressing gently, until the bread is golden brown and cheese is melted.
06 - Transfer to a cutting board, let rest 1 minute, slice, and serve warm.