Melt butter in a large pot over medium heat. Cook and stir onion and garlic in hot butter until softened and browned, 10 to 15 minutes.
Stir chicken broth, squash, curry powder, salt, cumin, and cayenne pepper into onion mixture. Bring mixture to a boil, reduce heat to low, and simmer until squash is tender, about 15 minutes. Remove from heat. Stir half-and-half and honey into squash mixture.
Pour squash mixture into a blender no more than half full. Cover and hold lid down; pulse a few times before leaving on to blend. Puree in batches until smooth. Ladle soup into serving bowls and top with a dollop of sour cream.