Melt the butter in a large pot over medium heat; cook the onion, garlic and thyme in the hot butter until the onion has softened, about 5 minutes. Add the squash and chicken stock; bring to a simmer and cook until the squash is tender, 10 to 15 minutes. Crumble the stock cube into the soup; season with cumin, allspice, salt and pepper; remove from heat.
Pour the soup into a blender, filling the pitcher no more than halfway. Hold the lid of the blender in place with a kitchen towel and carefully start the blender using a few quick pulses to get the soup moving before leaving it on to puree. Puree in batches until smooth and pour into a serving bowl. Alternately, you can use a stick blender and puree the soup in the pot.