How to Make It
Preheat oven to 350°.
Place chocolate and butter in a medium microwave-safe bowl; microwave at HIGH 1 minute, stirring until smooth. Cool 2 minutes.
Combine brown sugar and next 4 ingredients (through eggs) in a large bowl, stirring with a whisk. Add chocolate mixture, stirring to combine.
Weigh or lightly spoon flours into a dry measuring cup; level with a knife. Add flour, nuts, and xanthan gum to sugar mixture, stirring with a whisk. Pour batter evenly into a 9-inch round removable-bottom tart pan coated with cooking spray. Bake at 350° for 20 minutes or until a wooden pick inserted in center comes out clean. Cool 30 minutes in pan on a wire rack. Cut into 16 slices. Top with whipped topping.