Prepare the Blackberry Spread:

In a small bowl, gently mash the blackberries with a fork.
Stir in the balsamic vinegar and honey until well combined.
Assemble the Sandwich:

Spread the softened butter on one side of each bread slice.
On the unbuttered side of two slices, layer the Brie cheese slices.
Spoon the blackberry balsamic mixture over the Brie.
Place the other bread slices on top, buttered side facing out.
Grill the Sandwich:

Preheat a skillet over medium heat.
Grill the sandwiches for 3-4 minutes on each side, or until the bread is golden brown and crispy and the Brie is melted.
Serve:

Slice in half, garnish with fresh thyme or arugula if desired, and enjoy while warm.