Melt chocolate and butter over very low heat, stirring constantly (or heat in microwave bowl in microwave for 2-2 1/2 minutes, stirring after 1 minute, then stir until melted and smooth.) Cool slightly.
Beat eggs, sugar, and vanilla in large mixing bowl on high speed for 10 minutes. Blend in chocolate mixture on low speed. Add flour, beating just to combine. Stir in nuts, if using.