Preheat oven to 350°. Grease 13x9x2 inch baking pan or line with foil or parchment.
Add 4 squares chocolate and ⅔ cup butter to a large saucepan and cook over low heat until melted, stirring frequently. Remove from heat.
Add sugar, eggs, and vanilla and whisk until well blended. Stir in flour, baking powder, salt, and nuts if using, and mix just until blended. Spread into prepared pan.
Bake for about 30 minutes, or until brownies begin to pull away from the sides of the pan. (Do not overbake.)
Cool slightly; spread with Glossy Chocolate Frosting. Cool completely before serving.
Glossy Chocolate Frosting
Add 3 squares chocolate and 3 tablespoons butter to a large saucepan and cook over low heat until melted.
Stir in powdered sugar, milk, salt, and vanilla. Whisk until smooth.
Place pan of frosting in bowl of ice and water; continue beating until smooth, creamy, and of spreading consistency. Stir in nuts if using. (Moe prefers to “just step out on my porch and whisk, it's plenty cold in NY to firm up that icing.”)