Bring a large pot of water to a boil. Add about 2 tbsp. salt (the water should be as salty as the sea).
Meanwhile, melt butter in a small saucepan over low heat. Remove from heat and stir in calamansi juice, fish sauce, and 1 tsp. salt.
Cook noodles in boiling water until barely tender, 1 minute. Drain and return to pot.
Toss noodles with calamansi butter. Transfer half to a serving bowl and scatter with half of green onions. Pile in remaining noodles and scatter remaining onions on top.
*Unless you have a friend with a calamansi (aka calamondin) tree, you're unlikely to find this small, tart citrus that tastes like a cross between lime and kumquat with a faint floral bitterness. (Though Seafood City supermarket chain in California sometimes carries fresh calamansi.) So go online for bottled Sun Tropics pure calamansi (amazon.com). Or use our Test Kitchen fresh citrus substitute: Combine 3 tbsp. lime juice, 1 tbsp. Meyer lemon juice, and 1 tbsp. orange juice.
Find fresh pancit egg noodles and chow mein noodles at Asian markets. Find Megachef Premium fish sauce at amazon.com, or Red Boat fish sauce at well-stocked grocery stores.