Preheat the oven to 400°F (200°C). Spread the chestnuts on a baking sheet and roast for about 15-20 minutes until fragrant and slightly browned. Remove from the oven, let cool slightly, then chop coarsely.
In a large pot, melt the butter over medium heat while adding a splash of olive oil. Sauté the minced garlic until fragrant, about 1 minute, until it begins to turn golden and smells warm and inviting.
Add the chopped potatoes to the pot and stir to coat them evenly with the garlic and butter. Cook for about 5 minutes, allowing the potatoes to start softening and the edges to become slightly translucent.
Pour in the vegetable broth, bringing the mixture to a gentle boil. Reduce the heat to low, cover the pot, and simmer for about 20 minutes, or until the potatoes are completely tender and easily pierced with a fork.
Use an immersion blender to blend the soup directly in the pot until it becomes smooth and velvety. Alternatively, transfer the soup in batches to a blender, blend until smooth, then return it to the pot.
Add the roasted and chopped chestnuts to the blended soup. Stir well to combine and heat through for another 5 minutes, allowing the flavors to meld. Season with salt and pepper to taste.
While the soup is heating, toast additional chestnuts in a dry skillet over medium heat until golden and crispy, about 3-5 minutes, shaking the pan frequently.
Ladle the hot soup into bowls, then top each with the toasted chestnuts for added crunch and flavor. Optionally, drizzle with a little extra olive oil for a finishing touch.
Serve immediately and enjoy the rich, creamy texture contrasted with the crispy chestnut topping.
