Preheat the oven to 350°F. Grease an 8×8-inch metal baking pan with non-stick baking spray.
In a large microwave-safe bowl, combine the chocolate chips, butter, and unsweetened chocolate. Heat in the microwave for 1 minute then stir. Continue heating in 30-second sessions, stirring between each session, until the mixture is melted and smooth but not scorched. Remove from the microwave and let cool slightly.
1 cup Semisweet chocolate chips, 6 oz Unsalted butter, cut into pieces, 4 ounces Unsweetened Chocolate, chopped
In a large bowl, use an electric mixer to beat the eggs, egg yolk, and sugar on medium-high speed until light and thick, about 3 minutes. Beat in the vanilla extract.
1 1/2 cups Granulated Sugar, 3 large Eggs plus 1 Egg yolk, 1 teaspoon Vanilla extract
Stir in the cooled chocolate mixture with a rubber spatula.
Add the flour and salt, folding gently until combined.
3/4 cup All-Purpose Flour, 1/4 teaspoon Salt
Pour the batter into the prepared baking pan. Bake until a tester comes out with moist crumbs still attached, 35 to 40. Do not overbake.
Let cool to room temperature before slicing.