01 - In a small saucepan over medium heat, combine balsamic vinegar and honey. Simmer for 3 to 5 minutes until slightly thickened, then set aside to cool.
02 - Butter one side of each bread slice evenly. Arrange two slices, butter-side-down, on a clean work surface.
03 - Layer brie slices over the bread, sprinkle with shredded cheddar, and top with halved blackberries. Drizzle each assembled slice with one teaspoon of the prepared balsamic glaze.
04 - Top each arranged piece with another bread slice, butter-side-up, to enclose the fillings.
05 - Preheat a nonstick skillet or griddle over medium heat. Place sandwiches in the skillet and cook 3 to 4 minutes per side, pressing gently, until the bread is golden and the cheese fully melts.
06 - Remove from heat, let rest for 1 minute, then slice in half and serve warm.