Preheat the oven to 350°F. Line an 8x8-inch metal baking pan with foil or parchment and spray with nonstick cooking spray. Do NOT use glass or ceramic baking pans for this recipe, they will not allow the brownies to cook through the center.

In a large microwave-safe bowl, combine the chocolate chips, unsweetened chocolate, and butter. Microwave for 1 minute, then stir. Continue heating in 30-second bursts, stirring between each burst, until the mixture is melted and smooth but not scorched. Remove from the microwave and let cool slightly.

In a large bowl, use an electric mixer to beat the eggs, egg yolk, and sugar on medium-high speed until light and thick and ribbony, about 3 to 5 minutes or more depending on your mixer. Beat in the vanilla extract.

Using a rubber spatula, gently stir in the cooled chocolate butter mixture. Gently fold in the flour and salt until combined.

Pour the batter into the prepared baking pan. Bake until a tester comes out with moist crumbs still attached, about 35 to 40 minutes. Do not overbake the brownies as they will continue to bake once they’re out of the oven.

Let cool completely to room temperature before slicing. Serve at room temperature or chilled for an even fudgier texture.