Melt Chocolate & Butter: Using a double boiler or microwave, melt the semisweet chocolate, unsweetened chocolate, and butter together. Stir until smooth. Let it cool slightly.
Whisk Eggs & Sugar: In a separate mixing bowl, whisk together eggs, egg yolk, and granulated sugar for 3 minutes until light and fluffy.
Combine Mixtures: Gently fold in the melted chocolate mixture into the egg mixture. Stir in vanilla extract and salt.
Incorporate Dry Ingredients: Sift in the flour and fold gently using a spatula to maintain air bubbles. Avoid overmixing.
Add Optional Mix-ins: If desired, fold in chocolate chips or chopped nuts.
Bake: Preheat oven to 160°C (320°F). Pour the batter into a lined 8x8-inch baking pan. Bake for 28-30 minutes.
Cool Completely: Allow brownies to cool in the pan for at least 2 hours before slicing.