01 - Combine balsamic vinegar and honey in a small saucepan over medium heat. Simmer for 3 to 5 minutes, stirring occasionally, until slightly thickened. Remove from heat and allow to cool briefly.
02 - Spread softened butter on one side of each bread slice. Arrange two slices, butter-side-down, on a clean work surface.
03 - Layer brie slices evenly onto the bread, followed by the shredded cheddar cheese and blackberry halves. Drizzle each with approximately 1 teaspoon of the balsamic glaze.
04 - Place the remaining two bread slices on top, butter-side-up, to complete each sandwich.
05 - Heat a nonstick skillet or griddle over medium heat. Add the assembled sandwiches and cook for 3 to 4 minutes per side, pressing gently, until the bread is crisp and golden brown and the cheeses have melted.
06 - Remove sandwiches from the skillet. Let rest for 1 minute, then slice and serve while warm.