Preheat oven to 350°F. Line 9x9x2 inch pan with foil covering the bottom and extending up the sides.
Melt unsweetened chocolate and butter in a medium microwave safe bowl at medium power (50% or level 5) mixing at 30-second intervals until smooth and all of the chocolate is melted. Set aside.
In large bowl of electric mixer beat eggs, sugar, vanilla and salt at high speed for about 2-3 minutes or until light and creamy.
Blend in melted chocolate at low speed, stopping to scrape sides as needed.
Add flour just until incorporated. Spread into prepared pan.
Bake about 40-50 minutes or until top is puffed and cracked and toothpick inserted in center tests moist.
Allow to cool before cutting.