Preheat the oven to 375°. Lightly grease a 9-inch square baking pan and line the bottom with parchment paper. In a small bowl, whisk the flours with the salt. In a medium bowl, using an electric mixer, beat the butter with the sugar at medium speed until light and fluffy, about 3 minutes. Beat in the egg and vanilla. At low speed, beat in the dry ingredients along with the chopped chocolate and cocoa powder. Press the dough evenly into the pan. Bake for about 15 minutes, until set.