In a small saucepan, heat the balsamic vinegar over medium heat until it starts to simmer. Reduce the heat to low and let it simmer for about 10 minutes, or until it thickens slightly.

Remove from heat and let it cool.

Spread butter on one side of each slice of sourdough bread.

On the unbuttered side of four slices, spread a layer of blackberry preserves, followed by slices of brie cheese.

Drizzle a small amount of the balsamic reduction over the brie, then top with the remaining slices of bread, buttered side up.

Heat a skillet over medium heat. Place the assembled sandwiches in the skillet and cook for about 3-4 minutes on each side, or until the bread is golden brown and the cheese is melted.

Remove from skillet and let cool for a minute before slicing in half.