Preparation

• 将架子放在烤箱的中心，然后将烤箱加热到400°F。

  在一个中等大小的碗里,盖上温暖的佤邦的栗子ter and soak for 25 minutes. Drain and dry with paper towels. Score an X into the flat side of each chestnut, cutting all the way through the shell. Put them flat sides up on a rimmed baking sheet and roast until the Xs curl back into a crown shape and they are tender when squeezed, about 30 minutes. Peel while still warm, removing both the shell and the inner skin. If a shell resists peeling, spoon out the nutmeat instead. Roughly chop the chestnuts and set aside in a medium bowl.

  In a 6-quart pot, melt the butter with 2 Tbs. of oil over medium-low heat. Add the leeks and cook, stirring occasionally, until softened, 7 to 8 minutes. Add the chestnuts and 1 Tbs. of the thyme; cook until fragrant, about 1 minute. Add the broth and 1/2 tsp. salt; bring to a boil over medium-high heat. Reduce the heat to low and simmer until the chestnuts are extremely tender, about 30 minutes, skimming off any foam that rises to the surface. Let cool briefly. Working in batches, purée the soup, either in a regular blender or with a hand blender. Strain the soup through a fine sieve into a clean 4-quart pot. Set the soup back over medium-low heat and gently reheat. Season to taste with salt and pepper.

  Meanwhile, in an 8-inch nonstick skillet, heat the remaining 1 tsp. oil over medium heat. Add the prosciutto and cook, stirring frequently, until crisp, 2 to 3 minutes. Add the remaining 1/2 Tbs. thyme and cook 30 seconds more. Drain on a plate lined with paper towels.

  Ladle the soup into bowls and garnish with some of the prosciutto and thyme.