Heat oven to 350℉/177℃.
Spray an 8-inch square pan with baking spray. Set aside.
Combine chocolate and butter in a microwave safe bowl.
Microwave on high power for 30 seconds.
Stir well.
Continue to microwave in 15 second increments, stirring after each, until melted and smooth. Set aside.
Combine eggs, sugar, and vanilla in a large bowl.
Whisk until smooth.
Add chocolate.
Whisk until well combined.
Add flour.
Stir only until blended.
Pour into prepared pan.
Bake for 20 minutes, or until desired degree of doneness.
Cool in pan for 10 minutes.
Turn out onto cooling rack and allow to cool completely.