Poulet en Croûte

Name: Poulet en Croûte
Ingredients: 2 1/2 cups chopped button mushrooms (about 8 ounces); 1/3 cup bacon lardons (about 2 ounces) (from 1 to 2 thick-cut bacon slices); 2 medium shallots , chopped (about 1/2 cup); 1 1/2 pounds boneless, skinless chicken breasts , cut into 2-inch pieces; 2 1/2 teaspoons kosher salt , divided; 2 tablespoons all-purpose flour , plus more for work surface; 1/4 cup (2 ounces) Cognac; 1 1/4 cups (10 ounces) dry white wine; 1 (14-ounce) package frozen all-butter puff pastry sheet (such as Dufour), thawed; 8 ounces crème fraîche (about 1 cup); 3/4 cup chopped pitted green olives; 1 tablespoon finely chopped fresh tarragon; 1 teaspoon black pepper; Whole milk , for brushing