For the soup: In a large stock pot, heat butter and olive oil over medium heat. Add onion and garlic and gently sauté until onion is very soft, stirring regularly, about 12 to 15 minutes. Do not let the onion and garlic brown.

Add chicken stock and diced potatoes, and give it a stir. Increase heat to high and bring to a boil. Then lower heat to medium and cook for about 20 minutes, or until potatoes are completely tender. They MUST be fork-tender!

Using an immersion blender, very carefully blend the mixture until all chunks are broken down, and your desired consistency is reached. (Or very carefully transfer to a blender, in batches, and blend - then return blended mixture to the pot on the stove.) Stir in half & half, Parmesan, cumin, onion powder, garlic powder, cayenne, salt, and pepper.

Let soup warm back up for another 10-15 minutes before whisking in the sour cream.

For the herby compound butter: While soup is cooking, make the compound butter. In a small bowl, combine all ingredients until fully incorporated, and set aside.

To serve: Add a dollop of herby compound butter to individual bowls of soup and swirl it into the soup as it melts. Garnish with additional fresh rosemary and thyme if desired.