Preheat the oven to 350 degrees F.

Line a square 8x8 inch baking dish with parchment paper and spray with non-stick cooking spray.

Combine the chocolate chips, butter, and chopped unsweetened chocolate in a large microwave-safe bowl. Allow to cool.

In another bowl, whisk eggs, egg yolk, and sugar by hand or with an electric mixer for 3 minutes until light and thick. 

Add the vanilla and beat again until combined.

Add the slightly cooled melted chocolate mixture to the egg mixture. Stir to combine using a wooden or plastic spoon or spatula.

Add the flour and salt and gently stir it in just until no streaks of flour remain.

Stir in nuts and additional chocolate chips if desired.

Pour the brownie batter into the prepared baking dish and bake 30 to 33 minutes. Test with a wooden pick. Insert it in the center and remove. The brownies are done when the pick is mostly clean. Be careful not to overbake.

Cool completely before cutting. 

Store in an airtight container for up to 4 days.