Heat butter over medium heat in an enamel-coated cast-iron Dutch oven. Add onion and leeks; cook until softened, stirring frequently, for about 5 minutes. Add brown sugar and cook until bubbly. Stir in vinegar and cook for 2 minutes.

Add sweet potatoes, stock, smoked paprika, and smoked sea salt. Bring to a boil over medium heat, then reduce to a simmer. Cook uncovered for 30 minutes. Add cream and simmer for an additional 10 minutes, or until sweet potatoes are very tender.

Blend with an immersion blender, blender, or food processor. Season to taste with pepper and additional smoked sea salt.