In a saucepan, melt together chocolate and butter.
Take the saucepan off the heat and stir in sugar, vanilla and 2 eggs and. Mix well.
Add flour, salt and walnuts, if using. Stir to combine.
Pour the batter in a buttered and floured 8-inch (20 cm) square baking pan. Bake at 325 F (175 C) for about 30 minutes, or until a skewer inserted in the middle comes out clean.
Recipe Notes
* You can substitute unsweetened chocolate with 6 tablespoons (35 grams) unsweetened cocoa powder plus 2 Tablespoons (35 grams) unsalted butter or oil.