In a medium sized saucepan, add the balsamic vinegar. Bring to a boil over medium-high heat. Once boiling, reduce to a low simmer and simmer for about 10-15 minutes. The balsamic should reduce to about 1/3 cup (it will coat the spoon when it’s done).
In the meantime, heat a large skillet over medium heat. Lightly butter one side of each of the 4 slices of bread (toast bread in toaster if oil-free) and place bread butter-side-down in the skillet. Grill until bread is lightly browned. Remove bread from heat and set aside. In the same skillet on medium heat, add hempeh slices. Cook for two minutes on each side. Place hempeh on non-buttered sides of bread. Add tomato and avocado slices. Drizzle with a spoonful of balsamic reduction and top with a pinch of pink Himalayan salt and fresh-cracked black pepper to taste. Slice sandwiches in half and enjoy.