Sausage and Leek Savory Bread Pudding

Name: Sausage and Leek Savory Bread Pudding
Ingredients: 4 tablespoons unsalted butter; 8 cups 1/2-inch cubes brioche, crusts removed; 8 ounces sweet Italian sausage, casings removed; 1 cup finely chopped butternut squash; 4 leeks, white and light green parts only, thinly sliced; 1 tablespoon fresh thyme leaves, chopped; Kosher salt and freshly ground black pepper; 1 1/2 cups chicken broth; 1 cup heavy cream; 1/4 cup fresh flat-leaf parsley leaves, chopped; 3 tablespoons minced fresh chives, plus more for garnish; 4 large eggs; 2 cups grated Gruyere