Chicken Pot Pie with Acorn Squash

Name: Chicken Pot Pie with Acorn Squash
Ingredients: ½ cup (1 stick) chilled unsalted butter, divided; 1 pound skinless, boneless chicken thighs, cut into ¾-inch pieces; ¾ medium onion, chopped; 2 celery stalks, chopped; ½ teaspoon kosher salt, plus more; ¼ teaspoon freshly ground black pepper, plus more; ¼ cup sauerkraut; ¼ acorn squash, seeds removed, cut into ¾-inch pieces; 2 tablespoons plus 1 cup all-purpose flour; plus more for surface; 1 cup plus 2 tablespoons whole milk; 1 large egg yolk