01 - Combine balsamic vinegar and honey in a small saucepan over medium heat. 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. Place two bread slices buttered side down on a clean work surface.
03 - Arrange sliced brie evenly on the bread, then top with shredded white cheddar and halved blackberries. Drizzle each with one teaspoon of balsamic glaze.
04 - Close sandwiches with the remaining bread slices, butter side facing up.
05 - Heat a nonstick skillet or griddle over medium heat. Place sandwiches in the pan and cook for 3 to 4 minutes per side, pressing gently, until the bread is golden and the cheese has melted.
06 - Remove sandwiches from the pan and let rest for 1 minute. Slice and serve warm.