Melt butter and chocolate in top of a double boiler over boiling water, stirring constantly. Remove from heat; add 1/2 cup sugar and vanilla. Mix well; set aside.
Combine flour, baking powder, 1/2 teaspoon salt, pecans, and milk in a medium bowl; stir well. Add chocolate; stir well. Pour into a greased 8 - inch square pan.
Combine remaining sugar, salt, brown sugar, and cocoa; sprinkle over batter. Pour boiling water over topping; do not stir. Bake at 350° for 1 hour. Cool slightly; spoon into individual serving dishes. Top with whipped cream, if desired.