Combine the melted butter, sugar, and vanilla in a large bowl. Beat in the eggs, one at a time, mixing well after each, until thoroughly blended.
Sift the flour, cocoa powder in a bowl. Gradually stir flour mixture into the egg mixture until blended. Spread the batter evenly into the prepared baking dish. Sprinkle with walnuts or you can stir them in the batter.
Bake in preheated oven until an inserted toothpick comes out clean, 35 to 40 minutes. Remove, and cool pan on wire rack before cutting.