
Preheat oven to 350 degrees F. Line a metal 8-inch square pan with parchment paper.
In a large microwave-safe bowl, combine the chocolate and butter. Microwave in 30-second bursts, stirring between each burst, until the mixture is melted and smooth. Add the sugar to the hot butter mixture and whisk vigorously until combined. Allow to cool until just barely warm.
Add in eggs, egg yolk, and vanilla extract, and whisk for about 1 minute, or until very well combined. This helps create that shiny crust on top.
Use a rubber spatula to stir in flour, cocoa powder, and salt until just combined. Stir in the chocolate chips.
Pour into the prepared pan and smooth out. Bake in the preheated oven for 25 to 30 minutes, or until a toothpick inserted in the center comes out with moist crumbs. Let cool in pan 30 minutes before slicing.