In a Dutch oven, heat 2 tablespoons butter over medium heat; saute leeks and garlic until leeks are tender, 4-6 minutes.
Stir in sweet potatoes, ¾ teaspoon salt, cinnamon, cayenne and stock; bring to a boil. Reduce heat; simmer, covered, until potatoes are tender, 15-20 minutes.
Puree soup using an immersion blender or cool slightly and puree soup in batches in a blender; return to pan. Stir in cream and 2 teaspoons thyme; bring to a boil. Reduce heat; simmer, uncovered, 5 minutes.
Meanwhile, in a large skillet, melt remaining 2 tablespoons butter over medium heat. Add crab and the remaining salt and thyme; cook 5 minutes, stirring gently to combine. Top servings with crab mixture, and croutons if desired.
