1 teaspoon salt
1/4 cup fresh basil leaves
Trim off stem and blossom end of butternut squash. Halve lengthwise and scoop out and discard seeds and fibers. Cut into large chunks and carefully peel each chunk. Cut peeled chunks into 1-inch pieces; you will have about 4 cups. Set aside.
In a mini food processor or blender, combine shallot, garlic, ginger, chiles, 3 tablespoons water, and 1/2 cup of the cilantro and grind, pulsing and stopping often to scrape sides of container, until a fairly smooth paste forms. You will have about 1/4 cup bright green paste. Set aside.
Shake coconut milk can well. Spoon out 1/2 cup into a medium saucepan and bring to a gentle boil over medium heat. Cook, stirring occasionally, until thickened and fragrant, about 3 minutes.
Add curry paste and cook, mashing and stirring until paste is dissolved into coconut milk and heated through, 1 to 2 minutes. Add remaining coconut milk, 1/2 cup water, sugar, salt, and butternut squash. Increase heat to high and bring curry to a rolling boil. Stir well, reduce heat to maintain a gentle boil, and continue cooking until squash is tender and sauce is smooth and evenly colored a soothing green, about 15 minutes.
Meanwhile, cut all but a few of the basil leaves crosswise into thin strips. When curry is cooked, stir in basil strips and the remaining 1/4 cup of cilantro. Remove from heat and transfer to a serving bowl. Garnish with the reserved basil leaves and serve hot or warm.