Prepare the Blackberry Filling: In a small bowl, gently mash the blackberries with a fork. Mix in the balsamic glaze, honey, and black pepper. Let the mixture sit for 5 minutes to allow the flavors to meld.
Butter the Bread: Spread butter on one side of each bread slice.
Assemble the Sandwich: On the unbuttered side of two bread slices, layer slices of brie cheese, a spoonful of the blackberry mixture, and a sprinkle of fresh basil. Add goat cheese if using, then top with the second slice of bread (buttered side facing out).