Instructions

• Preheat the oven to 350°F (175°C). Grease a 9x13-inch pan and, if using, line it with parchment paper leaving a 1-inch overhang on two opposite sides for easy removal.
• Place 6 ounces chopped dark chocolate and 3/4 cup unsalted butter in a microwave-safe bowl. Microwave in 30-second intervals, stirring after each interval, until fully melted and smooth. Set aside to cool slightly (1–2 minutes).
• Stir the 1 1/4 cups granulated sugar and 1/3 cup packed dark brown sugar into the melted chocolate mixture until combined.
• Add the 3 large room-temperature eggs to the chocolate-sugar mixture one at a time, stirring or whisking after each addition until fully incorporated and the batter is glossy.
• In a medium bowl, sift or whisk together 3/4 teaspoon salt, 1 tablespoon cornstarch, 1/4 teaspoon baking powder, 1/4 cup cocoa powder, and 1 cup all-purpose flour.
• Add the dry ingredients to the chocolate-egg mixture and stir with a spatula or wooden spoon just until combined and no streaks of flour remain. Do not overmix.
• Stir 1 cup of the 1 1/2 cups semi-sweet chocolate chips into the batter (reserve the remaining 1/2 cup chips for the ganache). Pour the batter into the prepared pan and smooth the top with a spatula.
• Bake on the center rack for 40–45 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs but not wet batter. Rotate the pan once halfway through baking if your oven has hot spots.
• Remove the pan from the oven and transfer to a wire rack. Let the brownies cool completely in the pan (about 1 hour) before adding the ganache.
• To make the ganache, place the reserved 1/2 cup semi-sweet chocolate chips, 2 tablespoons unsalted butter, and 1/2 cup heavy cream in a microwave-safe bowl. Microwave in 20-second intervals, stirring between intervals, until the mixture is steaming and smooth.
• Pour the ganache over the cooled brownies and spread evenly with an offset spatula or the back of a spoon. Immediately sprinkle rainbow chips or sprinkles evenly over the ganache before it sets.
• Chill the brownies in the refrigerator for at least 1 hour to set the ganache. Use the parchment overhang to lift the brownies from the pan, slice, and serve.