In a saucepan, combine raspberries, sugar, lemon juice, and cornstarch. Cook over medium heat until thick and bubbly. Remove and cool.
Spread 1 TBSP raspberry onto each slice of cake. Place 3 slices of brie onto each slice of cake. Place 1 ounce of chocolate chunks on the bottom slice of cake. Place the top slice of cake onto the sandwich. Brush sandwich with melted butter. Grill at 325 degrees for 2-4 minutes. Flip and repeat. If bread is browning too quickly, reduce heat. Grill until cheese melts.