1 stick of unsalted butter
6 oz bittersweet chocolate (I also use dark chocolate 72%)
1 1/2 C sugar
3 large eggs
1/4 cocoa powder
1/2 t salt (use 1/4 t if use salted butter)
1/2 cup plus 2 T flour
1/2 cup of chopped walnuts or pecans (optional)

Preheat oven to 350 deg F. (325 deg on convection bake)
Line  8″ sq pan (or 9″ sq pan) with parchment, leaving an overhang on all sides.
Put butter and chocolate in a glass mixing bowl. Microwave on high 1 min. Stir until all chocolate is melted.
Stir in the sugar, then the eggs until combined.
Combine the cocoa, salt and flour in a separate bowl.
Fold in flour mixture till combined. Add nuts if desired.
Pour batter into pan. Bake  35 min or until a toothpick inserted in the center comes out with moist crumbs. (For my home oven set at 325 deg on conv bake, 8×8″ pan, 37 min was perfect, if 9×9 pan, 35 mins).
Let cool in pan 15 min, then lift from pan using the parchment and allow to cool on wire rack.
Cut with a plastic knife (while still slightly warm is best).
May serve warm with vanilla icecream.