In a medium saucepan over medium heat, melt 2 tbls butter until foaming. Saute garlic and ginger until soft (about 2 minutes - don't let it brown)
Pour soy sauce, vinegar, sugar, honey, water, chili sauce and orange juice into saucepan and bring to a gentle boil. Add cornstarch and water mixture and stir to dissolve. Reduce heat to a low simmer, occasionally stirring, for 20 - 25 minutes or until sauce has thickened. Taste and adjust seasoning if necessary.
Preheat oven to 425 degrees. Remove sauce from heat and let cool slightly, but keep a lid on the pan.
Melt the remaining 4 tbls butter. Place prawns in a single layer on two baking sheets brushed with butter, and brush the prawns with remaining melted butter. Season with pepper or to taste. Bake in hot oven for 8 to 10 minutes until pink and opaque. You can also grill the prawns on the BBQ.
Stir the sesame oil into the cooled sauce.
Place prawns on a serving platter and coat with the sauce. Garnish with the green onions and serve hot.