Preheat oven to 350°. Line an 8-inch square baking pan with parchment paper or nonstick aluminum foil.
Melt chocolate and butter in a large saucepan over very low heat. Stir in sugar, eggs, cherry balsamic, and vanilla.
Combine flour and next 2 ingredients; stir into chocolate mixture. Fold in chocolate chunks and nuts, if desired.
Spoon batter into prepared pan. Bake 20 to 22 minutes or until toothpick inserted in center comes out with moist crumbs clinging.
Cool completely in pan on wire rack.
Cover and chill until firm.