01 - In a small saucepan over medium heat, combine balsamic vinegar and honey. Simmer for 3 to 5 minutes until thickened, then set aside to cool slightly.
02 - Spread softened butter over one side of each sourdough bread slice.
03 - Arrange two bread slices, buttered-side-down, on a clean surface. Layer brie slices evenly, then top with shredded cheddar and blackberry halves. Drizzle each with about 1 teaspoon of balsamic glaze.
04 - Place the remaining bread slices on top, buttered-side-up, to close the sandwiches.
05 - Heat a nonstick skillet or griddle over medium heat. Add the sandwiches and cook for 3 to 4 minutes per side, pressing gently, until bread is golden brown and cheese is melted.
06 - Remove from the skillet, allow sandwiches to rest for 1 minute, slice, and serve warm.