Preheat oven to 350°F
To prep for baking, use butter to grease the interior of the 9x9 inch pan, extending butter up the sides
Combine the unsweetened chocolate and butter in a bowl, and melt together. You may microwave below 50% power at 30 second intervals, and mixing between intervals, until smooth. Or you may use a double boiler method at a lower heat. Once melted, set aside.
In a mixing bowl, combine the eggs, sugar, vanilla and salt at a high speed for 2-3 minutes or until light and creamy
Blend in the melted chocolate mixture at a low speed, stopping to scrape the sides as needed
Add flour until incorporated, and when mixed, pour into the greased pan
Bake for 40-50 minutes, or until the top if puffy and cracked. You may toothpick test to check if moist - it is best to not over bake brownies as they set while cooling
Allow to cool before cutting