1 cup semisweet chocolate chips
1 1/2 sticks (6 ounces) unsalted butter, cut into pieces
4 ounces unsweetened chocolate, chopped
1 1/2 cups granulated sugar
3 large eggs plus 1 egg yolk
1 teaspoon vanilla extract
3/4 cup (94 grams) all-purpose flour
1/4 teaspoon fine sea salt
Preheat the oven to 350°F. Line an 8×8-inch baking pan with foil and spray with nonstick cooking 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 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 and sugar on medium-high speed until light and thick, about 3 minutes. Beat in the vanilla extract. Stir in the cooled chocolate mixture with a rubber spatula. Add the flour and salt, folding gently until combined.
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. Serve at room temperature or chilled.