Preheat the oven to 350 degrees. Grease an 8 x 8 inch baking dish with butter. Set aside.

In a medium sized pot, melt the butter on the stove top over medium heat. Remove from heat, then whisk in brown and white sugar until smooth and creamy, about 1-2 minutes.

Add the eggs and egg yolk, one at a time, whisking well after each addition. Stir in the vanilla.

Add the cocoa powder, flour, and salt and stir until just combined. Stir in the chocolate chips.

Pour the mixture into the prepared pan and bake for 30-35 minutes (under done) or 35-40 minutes (baked through), or until the center is just set. A toothpick inserted in the center should come back with a few moist crumbs attached. Sprinkle with sea salt, if desired.

Allow to cool or eat directly out of the pan because you have no self control (at least, that's what I do.)