Line an 8x8-inch metal cake pan with parchment paper and spray with nonstick spray. Set aside.
In a stand mixer fitted with the whisk attachment (or using a hand mixer), beat the eggs, both sugars, and vanilla on medium-high speed until the sugars dissolve, about 1-2 minutes. Set aside.
3 eggs, 2/3 cup granulated sugar, ¼ cup brown sugar, 1 teaspoon vanilla extract
Cut the butter into tablespoon-sized pieces and finely chop the chocolate bars. Place both in a large microwave-safe bowl.
½ cup unsalted butter, 8 ounces semi-sweet chocolate bars
Microwave in 30-second increments, stirring between each, until melted and smooth. Be careful not to burn the chocolate. Stir to fully combine.
While the chocolate mixture is still warm, stir in the egg- sugar mixture until fully incorporated.
Add the flour, cornstarch, cocoa powder, and salt. Gently mix until just combined, do not overmix.
2/3 cup all-purpose flour, 1 Tablespoon cornstarch, ¼ cup cocoa powder, ¼ teaspoon salt
Fold in the chocolate chips.
¾ cup semi-sweet chocolate chips
Pour the batter into the prepared pan and smooth the top.
Bake for 30-35 minutes, or until a toothpick inserted in the center comes out mostly clean, a few moist crumbs are okay.
Allow to cool completely in the pan on a wire rack. Cut into 16 equal squares. Store in an airtight container at room temperature for up to 4 days.
Notes
Substitute chocolate chips for the chocolate bar if you don't have one.