Preheat your oven to 325°F (163°C). Line a 9x13-inch metal baking pan with parchment paper, leaving overhang on the long sides to form a sling. Lightly spray the parchment and exposed sides of the pan with nonstick spray.

In a large heat-safe bowl, melt the 1 cup (226 g) unsalted butter using the microwave or stovetop. Let it cool for 3–4 minutes until warm but not hot. Whisk in the granulated sugar and light brown sugar until the mixture looks glossy and thick.

Whisk the 3 eggs, 2 egg yolks, and 2 teaspoons vanilla extract into the butter-sugar mixture. Whisk vigorously for 30–45 seconds to help dissolve the sugar and create a shiny, crinkly top on the brownies.

Place a fine-mesh sieve over the bowl and sift in the cocoa powder, all-purpose flour, cornstarch, baking powder, and fine sea salt. This helps break up any cocoa clumps and keeps the batter smooth.

Using a spatula, fold the dry ingredients into the wet ingredients just until no dry patches remain. The batter will be thick and dark. Avoid overmixing so the brownies stay dense and fudgy rather than cakey.

Scrape the brownie batter into the prepared pan and smooth the top with an offset spatula or the back of a spoon, nudging the batter into the corners for even baking.

Bake at 325°F (163°C) for 22–28 minutes. Start checking at 22 minutes. The edges should look set and slightly pulled away from the pan, and a toothpick inserted near the center should come out with moist, fudgy crumbs but no wet batter. For very fudgy brownies, lean toward the shorter bake time.

Set the pan on a cooling rack and let the brownies cool completely in the pan, at least 1–1 1/2 hours. To speed this up, place the pan in the refrigerator for about 30 minutes. The brownies must be fully cool or the ganache will slide off and soak in too much.

Place the semi-sweet chocolate chips and 2 tablespoons softened butter in a medium heat-safe bowl. Heat the heavy cream in a small saucepan or microwave-safe cup until steaming and just beginning to bubble around the edges (do not boil). Pour the hot cream over the chocolate and let sit for 2–3 minutes. Add the vanilla extract and a pinch of salt, then whisk gently until smooth, glossy, and fully combined.

Pour the ganache over the cooled brownies and use an offset spatula to spread it into an even layer. Work fairly quickly, as the ganache will begin to thicken as it cools.

While the ganache is still soft and glossy, scatter the rainbow candy-coated chocolate chips or rainbow sprinkles evenly over the top. Press them very lightly with your palm so they adhere without sinking.

Refrigerate the pan for at least 1 hour, or until the ganache is set and sliceable. Use the parchment overhang to lift the brownie slab out of the pan onto a cutting board. Cut into 18–24 bars with a sharp knife, wiping the blade clean between cuts for neat, bakery-style edges.