Heat oven to 350°F/177℃.
Spray an 8 inch square pan with baking spray. Set aside.
In a small saucepan, combine chocolate and butter.
Place over low heat, stirring continuously, until melted and smooth. Set aside.
In the bowl of a stand mixer, combine eggs, sugar, salt, and vanilla.
Beat on medium speed until well blended, about 1 minute.
Add chocolate mixture and mix well.
Stir in flour on low speed just until combined.
Pour into prepared pan. (Wrap with cake strip, if using.)
Bake for 30 minutes.