01 - In a small saucepan over medium heat, combine balsamic vinegar and honey. Simmer for 3 to 5 minutes, stirring occasionally, until thickened. Remove from heat and allow to cool slightly.
02 - Spread softened butter on one side of each slice of sourdough bread. Arrange two bread slices, buttered side down, on a clean surface.
03 - Layer brie slices evenly over each bread base. Top with shredded cheddar and halved blackberries. Drizzle each with about 1 teaspoon of balsamic glaze.
04 - Place remaining bread slices on top, buttered side facing up.
05 - Preheat a nonstick skillet or griddle over medium heat. Place the assembled sandwiches in the pan and cook 3 to 4 minutes per side, pressing gently, until the bread is golden and crisp and the cheese is melted.
06 - Remove sandwiches from the heat. Let rest for 1 minute, then slice and serve immediately while warm.