Preheat oven to 350°F and place rack in middle of oven.
Butter a 9x13 baking pan.
In a heavy-bottomed saucepan over low heat, melt chocolate and butter. Once melted, remove from heat.
Add sugar, a cup at a time, mixing completely between additions.
Add eggs, one at a time, mixing completely between additions.
Add flour, mix thoroughly.
Add salt, mix thoroughly.
Add vanilla, mix thoroughly.
Pour into prepared pan. Sprinkle nuts on top, if using.
Bake 35-40 minutes, or until a toothpick inserted in the center comes out clean.