Potato Buns:

1. In the bowl of a stand mixer fitted with the dough hook, evenly combine the flours, dry milk powder, amylase, sugar, and salt.
2. In a small bowl, whisk together the water and yeast until the yeast dissolves. Whisk in the egg until homogenous. With the mixer running on low speed add the yeast mixture and stir for 2-3 minutes or until a rough dough forms.
3. Add the softened butter and mix for another 2-3 minutes or until the dough is smooth. Place the dough on a clean work surface and form into a ball. Place in a greased bowl, cover with plastic wrap and let rise for 1 hour at room temperature or until doubled in size.
4. Punch down the dough and divide it into 8 even pieces, about 120 grams each. Shape the dough pieces into balls, and place them on a sheet pan lined with parchment paper. Cover with greased plastic wrap and proof again at room temperature for 30-45 minutes or until doubled in size.
5. When the dough is almost done rising, preheat the oven to 375℉ / 190℃ .
6. Brush the tops of the dough with egg wash and bake for 15-17 minutes or until golden brown. Remove from the oven and immediately brush the tops with melted butter if you would like. Set aside.

Shack Sauce:

1. Place all ingredients in a small mixing bowl and mix until well combined. Season with salt to taste. Use immediately or store in the refrigerator until ready to serve.

Burger:

1. Portion the ground beef into 2 1/2 ounce balls and place on a sheet pan.
2. Set a medium sauté pan over medium heat and grease it lightly with cooking spray.
3. Once hot, place one ball in the middle of the pan and flatten it to a ¼-inch thick using the back of a spatula or a burger press. Season generously with salt and pepper to taste and sear for 1-2  minutes or until a golden brown crust forms. Flip, season on the other side and cook for another 1-2 minutes.
4. Add a slice of American cheese to the top of the burger patty and remove once the cheese has melted. Repeat this process with another beef patty but this time add one slice of white cheddar cheese on top. Keep warm while you repeat cooking the remaining burgers.

Assembly:

1. Heat a skillet over medium heat and melt a tab of butter. Cut a potato bun in half and toast, cut side down in the butter until golden brown. Repeat toasting the remaining buns, adding more butter to the skillet as needed.
2. Add the Shack sauce to the top and bottom bun. Add a patty with American cheese followed by a patty with white cheddar on top of the bottom bun. Add a slice of tomato, season the tomato with salt to taste, add a piece of lettuce and crown with the top bun. Serve immediately.