Peel and cut the onion into cubes. Heat the clarified butter in a large saucepan over medium heat. Add the onion and roast for 3-4 minutes until softened. Roughly dice the chestnuts and add to the saucepan. Stir in the sugar and continue cooking over medium heat for 3-4 minutes until the mixture is caramelized.
Pour in the white wine and let it simmer for 2-3 minutes until reduced by about half. Add the broth and bring to a simmer.
Simmer for 20 minutes.
Remove from heat and stir in the cream and sour cream until well combined. Return to low heat and warm through for 2-3 minutes, stirring occasionally, without allowing it to boil.
Remove from heat and stir in the amaretto. Season with salt, pepper, and a pinch of nutmeg. Transfer to a blender and puree until smooth. Return to the saucepan and taste, adjusting salt and pepper as needed.
Pour the milk into a small saucepan and place over low heat. Using a flat whisk, stir constantly for 3-4 minutes until foamy and warmed through.
Pour the chestnut soup into coffee cups and top with the milk foam. Serve immediately.