01 - In a small saucepan over medium heat, mix balsamic vinegar and honey. Simmer for 3 to 5 minutes, stirring occasionally, until the mixture thickens. Allow to cool slightly.
02 - Spread softened butter evenly on one side of each bread slice. Position two slices, buttered side down, on a clean work surface.
03 - Layer sliced brie cheese evenly atop the bread, sprinkle with shredded cheddar, and scatter halved fresh blackberries over cheese. Drizzle each sandwich with 1 teaspoon of the balsamic glaze.
04 - Top with the remaining bread slices, buttered sides facing outward.
05 - Heat a nonstick skillet or griddle over medium heat. Place sandwiches in the pan and cook for 3 to 4 minutes per side, pressing gently, until bread is golden and cheese is melted.
06 - Remove sandwiches from the skillet and allow to rest for 1 minute. Slice and serve warm.