Preheat oven to 350 degrees F (175 degrees C).

Line a 13x9-inch pan with foil, extending the ends over the sides for easy removal.

Spray the foil-lined pan with cooking spray to prevent sticking.

In a large microwaveable bowl, combine unsweetened chocolate and butter.

Microwave on HIGH for 2 minutes, or until butter is melted.

Stir until the chocolate is completely melted and the mixture is well blended.

Stir in sugar and cinnamon until well combined.

Add the beaten eggs and vanilla extract; mix well.

Blend in flour and orange zest until just combined.

Stir in chocolate chunks and chopped walnuts.

Spread the brownie batter evenly into the prepared pan.

Bake for 30 to 35 minutes.

Check for doneness by inserting a toothpick into the center; it should come out with fudgy crumbs (avoid overbaking).

Cool completely in the pan.

Using the foil handles, lift the brownie slab from the pan before cutting it into squares to serve.