Preheat oven to 325°F. Line an 8x8 metal pan with parchment paper and set aside.

Add butter to a heatproof, medium bowl and melt the butter in the microwave. Immediately add the chopped chocolate, cocoa powder, and vanilla to the melted butter and stir until the chocolate is completely melted and the mixture is smooth and shiny. Set the chocolate mixture aside to cool slightly while you move on to the next step.

In a small bowl, whisk together flour, cornstarch, and salt. Set aside.

In a the bowl of a stand mixer using the whisk attachment (or in a large bowl using a handheld electric mixer), beat together eggs and sugar on medium-high speed until the mixture is light, fluffy, and pale yellow, about 2 minutes.

With the mixer running on medium speed, slowly pour the slightly cooled chocolate mixture into the egg mixture. Continue to beat the mixture on medium speed until well combined, about 30 seconds.

Remove the bowl from the stand mixer (if using) and add the flour mixture. Using a rubber spatula or wooden spoon, gently fold the flour mixture into the batter. Continue to fold until all flour is combined, being careful not to over mix the batter.

Pour the batter into the prepared pan and use the spatula or spoon to gently smooth the top of the batter.

Bake the brownies for 50 minutes or until they have risen and a paper-thin crust has formed atop the brownies. The brownies are finished baking when a toothpick inserted into the center of the pan comes out clean or with just a few moist crumbs. 

Place the pan on a wire rack and allow the brownies to cool completely in the pan before slicing and serving.