1/2 cup (1 stick) unsalted butter, cut into pieces, plus more for pan
1 bag (12 ounces) semisweet chocolate chips
1/2 cup granulated sugar
1/2 cup packed light-brown sugar
1/2 teaspoon salt
2 large eggs
3/4 cup all-purpose flour
1 cup marshmallows
1/2 cup chopped walnuts