2 lbs. ground beef
Sea salt and freshly ground black pepper,
1 tsp. cornstarch
1 cup hot veal demi-glace
2 tbsp. extra-virgin olive oil
1⁄4 cup madeira or red wine
4 tbsp. unsalted butter, at room temperature
4 truffled brioche buns or hamburger buns
10 oz. duck foie gras, sliced into 1⁄2"-thick slabs
1 oz. black truffle, thinly sliced
1. Shape beef into four 1"-thick patties; transfer to a plate. Season patties with salt and pepper; set aside.
2. Heat oven to 200˚. In a bowl, whisk cornstarch with 2 tbsp. demi-glace; set aside. Heat oil in a 12" cast-iron skillet over medium-high heat. Add burgers; cook, flipping once, until browned and rare, about 6 minutes. Place burgers on a baking sheet; bake until medium rare, about 5 minutes. (This helps keep the burgers juicy.) Transfer to a plate. Meanwhile, discard fat from skillet; return to medium-high heat. Add madeira; cook, scraping browned bits from pan, until liquid is nearly evaporated, 1–2 minutes. Add remaining demi-glace; boil. Reduce by a third, 1–2 minutes. Stir in cornstarch mixture; cook until thickened, about 1 minute. Set sauce aside.
3. Butter buns with 2 tbsp. butter; toast on a baking sheet in oven. Heat a 12" skillet over medium heat. Season foie gras with salt and pepper; sear, flipping once, until browned, about 2 minutes. Transfer foie gras to paper towels. Whisk fat from skillet into reserved sauce.
4. Melt remaining butter in a 10" skillet over medium heat. Add truffles; cook until hot, about 30 seconds. To assemble burgers, place a patty on each bun bottom; drizzle with a little sauce. Top each burger with a slice of foie gras and a few truffle slices. Sprinkle salt over the top; add top half of bun.
Pairing Note The Brezza Cannubi 2004 ($84), a barolo from Italy's Piedmont region, has an acidity and dry finish that stand up to a rich burger. —Ania Zawieja
MAKES 4 BURGERS