In a medium saucepan, heat butter over medium-heat until melted. Cook and stir onion and garlic in hot butter about 7 minutes or until onion is tender. Stir in tomato, chile peppers, sugar, cocoa powder, chili powder, and salt. Bring to boiling; reduce heat. Simmer, covered, for 10 minutes. Remove from heat; set aside.
Tear four 18x12-inch pieces of parchment paper; fold each in half crosswise and crease. Open up again. On half of one parchment sheet, arrange one-fourth of the chicken. Top with one-fourth of the sauce mixture. Top with one-fourth of the sliced green onions. To make packet, fold parchment paper over chicken. Crimp and fold edges to seal; twist corners. Repeat to make 4 packets.
Fill a large Dutch oven with water to a depth of 1 inch. Bring water to boiling. Place a steamer basket in the Dutch oven. Arrange chicken packets in a single layer in the steamer basket. Cover and steam for 15 minutes or until chicken is no longer pink.
To serve, if desired garnish with slivered green onion and serve with hot cooked brown rice.