Preheat oven to 375 degrees. cut squash in half, scoop out seeds, brush cut surfaces with oil and set them face down on a baking sheet. Bake until very tender, about 30 to 45 minutes, depending on how large the squash is.
While this is baking, bring the milk, cinnamon stick, cloves and garlic to a simmer. Remove from heat and let the milk infuse while squash is baking.
When squash is done, remove it from oven and scoop out the cooked flesh which you should transfer to a food processor or blender. Strain the infused milk to remove the spices and garlic and blend infused milk with the squash to make a puree. Beat in butter if you wish and season to taste with salt and pepper. You can make this in advance and reheat in a double boiler.