Step 1: Prepare the Blackberry Balsamic Reduction: In a small saucepan, combine blackberries, balsamic vinegar, and sugar (if using). Bring to a simmer over medium heat, then reduce heat to low and cook for 5-7 minutes, stirring occasionally, until blackberries break down and the sauce thickens slightly. Mash the berries gently with a fork or potato masher. Remove from heat and set aside.
Step 2: Prepare the Bread: Butter one side of each bread slice evenly. This buttered side will be the outside of your sandwich.
Step 3: Assemble the Sandwiches: On the unbuttered side of two bread slices, spread a generous layer of the blackberry balsamic reduction. Top with half of the cheese. Place the remaining cheese on the unbuttered side of the other two bread slices. Bring the two halves together to form two sandwiches.
Step 4: Cook the Grilled Cheese: Heat a non-stick skillet or griddle over medium-low heat. Place the sandwiches, buttered-side down, in the hot pan. Cook for 4-6 minutes per side, or until the bread is golden brown and crispy, and the cheese is completely melted and gooey. If necessary, cover the pan for a minute or two during the last few minutes of cooking to help the cheese melt through.
Step 5: Serve: Carefully remove the sandwiches from the pan, slice in half diagonally, and serve immediately. Enjoy your gourmet grilled cheese!