Preheat oven to 350.
Line 8×8 baking pan with parchment paper/nonstick foil long enough to hang over on two sides, making it easier to remove from pan once baked. Spray pan and paper.
In microwave, melt chocolate and butter until smooth. Allow to cool slightly.
In a separate bowl, beat eggs, vanilla with the sugar. Slowly add in chocolate mixture until well combined.
In small bowl, sift together cocoa, salt and flour. Add mixture to rest of batter. Do not over mix.
Pour batter into prepared baking pan.