Beat in flour, cocoa powder and xanthan gum until well combined.Beat in melted coconut oil and melted chocolate until thoroughly mixed, and then stir in chopped nuts.Pour batter into prepared pan and smooth the top with a knife or offset spatula.Bake 30 to 35 minutes, until edges are lightly browned and center is just barely cooked through.Let cool in pan 10 minutes, then run a sharp knife around to loosen, and remove sides.Let cake cool completely before slicing.