Heat oven to 350℉/177℃.
Spray an 8-inch square pan with baking spray. Set aside.
Place chocolate and butter in a microwave safe bowl.
Microwave on high power for 30 seconds.
Stir well.
Microwave in 15 second increments, stirring well after each, until melted and smooth. Set aside.
In a large bowl, whisk together eggs, sugar, and vanilla.
Whisk in chocolate mixture.
Stir in flour.
Pour into prepared pan.
Bake for 30 minutes, or until desired degree of doneness.
Let cool in pan for 5-10 minutes before turning out.