Preheat your oven to 350°F (175°C). Line a 9×13-inch metal baking pan with parchment paper, leaving some overhang on the long sides for easy lifting. Lightly grease the parchment and pan sides with butter or nonstick spray.

In a large mixing bowl, whisk together the melted unsalted butter, granulated sugar, and light brown sugar until thick and glossy, about 1 minute. Add the eggs and extra egg yolk one at a time, whisking well after each addition. Whisk in the vanilla extract and the milk or cream until smooth and slightly shiny.

In a separate medium bowl, sift or whisk together the unsweetened cocoa powder, all-purpose flour, cornstarch, baking powder, and fine sea salt. Break up any lumps of cocoa with the whisk so the mixture is smooth and even.

Sprinkle the dry mixture over the wet mixture. Switch to a rubber spatula or wooden spoon and gently fold the dry ingredients into the wet using broad, sweeping motions, scraping the bottom and sides of the bowl. Stop as soon as no dry streaks remain; the batter will be thick and fudgy.

Spread the brownie batter evenly into the prepared 9×13 pan, smoothing the top and pushing it into the corners. Bake for 18–24 minutes, or until the edges are set and just starting to pull away from the sides, the center looks set but still soft, and a toothpick inserted about 2 inches from the edge comes out with a few moist crumbs (not wet batter). Do not overbake.

Place the pan on a wire rack and let the brownies cool completely in the pan for at least 1 hour, until cool to the touch. For the cleanest ganache layer, chill the cooled brownies in the refrigerator for an additional 30 minutes before frosting.

Place the semi-sweet chocolate chips (or chopped chocolate) in a medium heat-safe bowl. In a small saucepan over medium-low heat, warm the heavy cream just until it barely starts to simmer, with small bubbles around the edges but not a full boil. Pour the hot cream over the chocolate, ensuring all the chocolate is submerged, and let sit undisturbed for 2–3 minutes. Add the room-temperature butter, light corn syrup (if using), and a pinch of fine sea salt. Stir gently from the center outward until the mixture is smooth, glossy, and fully melted. If small bits of chocolate remain, microwave the bowl for 5–10 seconds and stir again.

Pour the warm ganache over the fully cooled brownie layer and spread it into an even layer, nudging it into the corners with a spatula. Let the ganache sit at room temperature for 5–10 minutes until it thickens slightly but is still soft. Sprinkle rainbow candy sprinkles or rainbow chocolate chips evenly over the top and press them very gently so they adhere without sinking into the ganache.

Chill the frosted brownies in the refrigerator for 1–2 hours, or until the ganache is set and firm to the touch. Use the parchment overhang to lift the entire slab of brownies out of the pan onto a cutting board. With a long, sharp knife, cut into 24 small bars (or 12–18 larger pieces), wiping the knife clean with a warm, damp cloth between cuts for sharp edges. Serve slightly chilled or at room temperature.