Preheat oven to 375 degrees. Line bottom of 8-inch springform pan with parchment paper. Spray parchment paper and sides of pan with cooking spray.
Combine 2/3 cup brown sugar, chocolate, cocoa and boiling water in a small bowl. Stir until chocolate is melted. Let cool to room temperature. Whisk in egg yolks. Set aside.
Finely grind almonds in food processor or clean coffee grinder. Add xanthan gum. Set aside.
Using electric mixer, beat egg whites and cream of tartar until soft peaks form. Add 1 tablespoon brown sugar, vanilla extract and almond extract. Beat until stiff but not dry.
Carefully whisk almond mixture into chocolate mixture. Fold in egg whites in two additions.
Transfer batter to prepared springform pan. Wrap bottom in aluminum foil to prevent leaking. Place in larger pan of boiling water so that water comes up one inch on outer sides of springform pan.
Bake 35 minutes or until toothpick inserted into center comes out clean. Cool cake in pan on rack. Dust with powdered sugar. Cut around edge of cake to loosen from pan edges. Release pan sides. Cut into 10 slices.