Preheat the Oven Begin by preheating your oven to 350°F (175°C). Prepare your baking pan by lining an 8x8 inch metal baking pan with parchment paper, leaving some overhang for easy removal later.
Beat the Eggs and Sugar In a stand mixer, combine 2 large room temperature eggs, 1 cup of granulated sugar, and 1 teaspoon of pure vanilla extract. Beat for 3-4 minutes until pale and creamy.
Melt the Butter Melt 10 tablespoons of salted butter in a saucepan over low heat. Remove from heat and whisk in ½ cup of icing sugar, ⅔ cup cocoa powder, and ¼ teaspoon of salt.
Combine Mixtures Pour the warm cocoa mixture into the egg and sugar mixture, and beat for another 30 seconds until the batter is smooth and shiny.
Add the Flour and Chocolate Chips Gently fold in ½ cup of all-purpose flour and mix until just combined. Add ½ cup of dark chocolate chips.
Pour and Bake Transfer the batter into the prepared pan and bake for 20-22 minutes. Check for doneness by inserting a toothpick.
Cool and Cut Allow to cool completely before refrigerating for at least 1 hour. Slice into squares.