01 - In a small saucepan over medium heat, combine balsamic vinegar and honey. Simmer for 3 to 5 minutes, stirring frequently, until the mixture thickens to a glaze consistency. Allow to cool slightly.
02 - Spread softened butter evenly on one side of each slice of sourdough bread. Arrange two slices butter-side-down on a clean work surface.
03 - Place sliced brie evenly over the bread, then top with shredded sharp white cheddar and halved blackberries. Drizzle each with 1 teaspoon of the prepared balsamic glaze.
04 - Top with the remaining bread slices, butter-side-up, to form two complete sandwiches.
05 - Preheat a nonstick skillet or griddle over medium heat. Add the sandwiches and cook for 3 to 4 minutes per side, pressing gently, until the bread is golden brown and the cheeses have melted.
06 - Remove sandwiches from the pan and allow them to rest for 1 minute. Slice in half and serve warm.