Heat the ¼ cup of olive oil and 4 tablespoons of butter in a large, heavy-bottomed soup pot or Dutch oven over medium-high heat until melted and foaming.

Add the sliced leek, chopped yellow onion, and whole garlic cloves to the hot fat. Cook, stirring occasionally, until the vegetables are completely soft, translucent, and highly fragrant, about 10 minutes.

Add the chopped butternut squash to the pot.

Aggressively season the squash and aromatics with the Autumn spice blend, chili powder, kosher salt, and coarse black pepper. Stir continuously for 2 minutes to bloom the dry spices in the residual fat.

Pour the ⅓ cup of dry sherry into the pot to deglaze. Simmer vigorously until the sharp alcohol scent burns off and almost all of the liquid is completely absorbed by the vegetables.

Pour the 1 quart of chicken broth into the pot. Reduce the heat to medium-low, cover partially, and simmer until the butternut squash is incredibly soft and yields entirely to the back of a spoon, about 20 minutes.

Remove the pot from the heat. (If using a standard blender or food processor, allow the soup to cool slightly to prevent thermal expansion blowouts).

Using a professional immersion blender directly in the pot, aggressively puree the softened squash and broth until it transforms into a perfectly smooth, opaque, velvety liquid emulsion.

Return the pureed soup to low heat.

Slowly stir in the ¾ cup of heavy whipping cream. Do not allow the soup to boil once the cream is added, or the dairy may break.

Taste the bisque and adjust the seasoning with additional salt and pepper if necessary. Serve piping hot.