The salsa and balsamic mayo can be made a day or two in advance; refrigerate in separate containers with plastic wrap directly on the surface. You may have a little salsa verde left over, which would be great on garlic bread or stirred into scrambled eggs.
For the salsa verde
1 thick slice day-old crusty white bread
2 tablespoons water
1 tablespoon capers
3 anchovy fillets
1 small clove garlic
Leaves from 8 to 10 stems flat-leaf parsley (1 packed cup)
½ teaspoon white wine vinegar
5 tablespoons olive oil
Kosher or sea salt
For the burgers
1 pound very cold ground beef (preferably 92 percent lean)
½ cup packed freshly grated Parmigiano-Reggiano cheese
Kosher or sea salt
Freshly ground black pepper
1 tablespoon olive oil
2 tablespoons unsalted butter
4 hamburger buns, preferably potato
For the balsamic mayonnaise
¼ cup mayonnaise
2½ teaspoons aged balsamic vinegar
For the salsa verde: Place the bread in a medium bowl. Pour the water over it and let it soak in for a minute or two, then tear the bread, letting the pieces fall into a food processor. Drain the capers and anchovy fillets.
Cut a few slices off the garlic clove (to taste) and add to the food processor, along with the parsley leaves, capers, anchovies, vinegar, oil and a pinch of salt. Puree until fairly smooth, stopping to scrape down the sides of the bowl as needed. The yield is about 1 cup.
For the burgers: Place the chilled ground beef in a mixing bowl. Fold in the cheese until well incorporated. Season lightly with salt and pepper. Divide the mixture into 4 equal portions and form each into a ¾-inch-thick patty.
Heat the oil in a large skillet over medium-high heat. Once the oil simmers, add the burger patties and cook for 2 to 3 minutes per side, until well seared and just cooked through (medium-rare). Transfer to a cutting board to rest while you prepare the buns.
Melt the butter, then use it to spread on the inside of the hamburger buns. Toast in a toaster oven until golden brown.
For the balsamic mayonnaise: Stir together the mayonnaise and balsamic vinegar in a small bowl, until well incorporated.
To serve, spread each bottom bun with one-quarter of the balsamic mayo. Place a burger on top, then spread salsa verde over each burger and finish with the top buns. Serve warm.
Serves 4 to 8
Nutrition | Per half burger (using half the salsa verde): 400 calories, 17 g protein, 27 g carbohydrates, 25 g fat, 9 g saturated fat, 75 mg cholesterol, 510 mg sodium, 1 g dietary fiber, 4 g sugar
Adapted from "The Shake Shack Cookbook: Recipes and Stories," by Randy Garutti and Mark Rosati (Clarkson Potter, 2017).