Preheat the oven to 350 degrees F. Spray an 8-inch baking pan with nonstick cooking spray.
In a small bowl, whisk together the flour, cocoa powder and kosher salt. Add the granulated sugar, melted butter, vanilla and eggs and whisk until a few lumps remain. Stir in the chocolate chunks.
Pour the batter into the prepared pan and sprinkle with the turbinado sugar and flake salt. Bake until a toothpick inserted in the center comes out with just a few moist crumbs, about 25 minutes. Allow to cool completely in the pan.
Cut the brownie in half. Using the tip of a knife, slice in angles making W's with the cut to form triangular pieces. Remove each piece and decorate with frosting and sprinkles as desired.