Melt chocolate and butter in a double boiler over medium heat (or in a saucepan over very low heat), stirring frequently. Remove from heat and stir in sugar and vanilla extract. In a large bowl, whisk together teff flour, GF Baking Mix, guar or xanthan gum, baking powder, and salt. Add chocolate mixture to flour mixture and blend until completely incorporated. Mix in eggs and walnuts, if using. Batter should be very thick and a bit grainy.