Prep the Oven & Pan: Preheat your oven to 350°F (175°C). Lightly grease a 13x9-inch baking pan, or line it with parchment paper leaving a slight overhang for easy removal later.
Melt Chocolate & Butter: Place the unsweetened chocolate squares and butter into a large microwave-safe bowl. Microwave on HIGH power for 2 minutes, or until the butter is completely melted. Remove the bowl and stir the mixture vigorously until the chocolate melts entirely and the mixture is perfectly smooth.
Add Sugar: Stir the granulated sugar into the warm melted chocolate mixture until it is well blended.
Whisk in Eggs & Vanilla: Add the eggs and vanilla extract to the bowl. Whisk thoroughly until the batter becomes beautifully glossy and smooth.
Fold in Flour: Add the all-purpose flour (and the chopped nuts, if you are using them). Use a silicone spatula to gently fold the dry ingredients into the wet mixture. Be careful not to overmix; stir just until the flour streaks disappear.
Bake: Pour the brownie batter into the prepared baking pan and spread it out evenly into the corners. Bake in the preheated oven for 30 to 35 minutes.
Check for Doneness: Insert a toothpick into the center of the pan. If it comes out with a few fudgy, moist crumbs attached (not wet batter), the brownies are perfectly baked.
Cool & Serve: Place the pan on a wire rack and allow the brownies to cool completely. Once fully cooled, lift them out using the parchment paper overhang, cut into squares, and serve.
