In a small bowl, combine the softened butter, finely chopped dried porcini mushrooms, minced parsley, minced chives, minced garlic, and lemon juice. Mix well until all ingredients are combined. Season with salt and pepper to taste. Set aside.

Preheat your oven to 400°F (200°C). Season the filet mignon steaks generously with salt and freshly ground black pepper on both sides.

In a skillet over medium-high heat, add 1 tablespoon of black truffle oil. Once hot, sear the filet mignon for about 3-4 minutes on each side until a golden-brown crust forms.

Remove the skillet from heat and press the chopped roasted chestnuts onto the top of each filet, ensuring an even coating.

Transfer the skillet to the preheated oven and roast for an additional 8-10 minutes, or until the internal temperature reaches your desired doneness (130°F for medium-rare). Once done, remove the skillet from the oven and allow the steaks to rest for 5 minutes.

To serve, place each filet mignon on a plate and top with a generous dollop of the whipped porcini butter. Drizzle with the remaining black truffle oil and garnish with fresh thyme if desired.