Preheat the oven to 350°F. Spray an 8-inch baking pan with nonstick cooking spray.
In a small bowl, whisk together the flour, cocoa powder and salt. Set aside.
Melt the butter in a medium bowl using the microwave in 20 second increments until just melted, about 40 seconds. Whisk in the sugar and vanilla until incorporated. Whisk in the eggs, one at a time, until smooth.
Using a rubber spatula fold in the flour mixture until only a few streaks of dry ingredients remain. Add the chocolate and fold a few more strokes to incorporate.
Pour the batter into the prepared pan and sprinkle with the flake salt and turbinado sugar. Bake for 28 minutes or until a toothpick inserted in the center comes out with just a few moist crumbs. Let cool completely in the pan.