Preheat oven to 350°F (177°C) and line a 9x9-inch baking pan with parchment paper.

In a large bowl, whisk together melted butter and chocolate chips until smooth.

Stir in the sugar, eggs, egg yolks, and vanilla extract until glossy and well combined.

Sift in the cocoa powder and flour, then gently fold until just incorporated.

Pour the batter into the prepared pan and bake for 35–40 minutes, or until a toothpick comes out with a few moist crumbs.

Let the brownies cool completely in the pan.

In a mixing bowl, whisk together the eggs, egg yolks, and cornstarch until smooth. Set aside.

In a medium saucepan over medium heat, combine the milk, sugar, salt, and a pinch of cinnamon (optional but delicious). Whisk gently until the mixture is steaming but not boiling.

Carefully scoop out 1 cup of the hot milk mixture and slowly drizzle it into the egg mixture, whisking constantly to temper the eggs and prevent them from scrambling.

Gradually pour the tempered egg mixture back into the saucepan with the remaining milk, whisking continuously as you add it.

Cook over medium heat, whisking constantly, until the custard thickens—about 3 to 5 minutes.

Once thick and pudding-like, remove from heat and stir in the vanilla extract.

Pour the warm custard over the cooled brownie layer and spread it evenly with a spatula.

Chill in the refrigerator for at least 30 minutes to set before adding the ganache.

In a small saucepan, heat the heavy cream until it just begins to simmer.

Pour the ganache over the custard layer and spread evenly with an offset spatula.

Chill the assembled brownies for 2–4 hours, or until fully set.