Melt chocolate with butter in a saucepan over very low heat, stirring constantly until smooth. Remove from heat, Mix flour, salt, and baking power. Beat eggs thoroughly. Gradually beat in sugar. Blend in chocolate mixture and vanilla extract. Add flour mixture, stir well. Stir in nuts if desired. Spread in greased 8-inch square pan. Bake at 350 °For 25 minutes (for moisten chewy brownies) or about 30 minutes (for cake-like brownies). Cool in pan. Cut into squares or bars.