01 - Combine balsamic vinegar and honey in a small saucepan over medium heat. Simmer gently for 3 to 5 minutes, stirring occasionally, until glaze thickens. Set aside to cool slightly.
02 - Spread softened butter on one side of each bread slice.
03 - Lay two slices of bread butter-side-down on a clean work surface. Top each with sliced brie, then sprinkle with shredded cheddar. Arrange blackberry halves evenly on top. Drizzle each sandwich with about 1 teaspoon balsamic glaze.
04 - Place remaining bread slices over the fillings, butter-side-up.
05 - Heat a nonstick skillet or griddle over medium heat. Add the sandwiches and cook for 3 to 4 minutes per side, pressing gently with a spatula, until the bread is golden and cheese melts.
06 - Remove sandwiches from the heat and let them rest for 1 minute. Slice in half and serve warm.