Heat a large saute pan over medium-high heat, add the oil and swirl to coat bottom of pan. Add shallots, ginger bud, garlic scapes, chilies and sauté until softened and aromatic, 1 to 2 minutes.
Add the prawns, mangoes and sugar, and sauté one minute. Add lemongrass, fish sauce, chicken stock, lemon juice and cook until liquid reduced by 1/2, about 2 minutes.
Add butter, toss and taste for seasoning.
Transfer prawns to serving plate, top with mango sauce and garnish with garlic scapes or chives.