Preheat oven to 350°F / 175°C.
Grease a 9×13″ pan with butter or cooking spray.
Make sure the oven rack is in the middle of the oven.
In a small microwave-safe bowl, melt butter and chocolate in the microwave (or over a double boiler). Allow to cool slightly.
Measure flour, salt, and baking powder into another small bowl.
In a larger mixing bowl, beat eggs thoroughly. A whisk is fine.
Gradually add sugar.
Blend the chocolate mixture and vanilla into the egg/sugar mixture.
Add flour mixture, and stir well.
Spread in the greased 9×13″ pan and bake for 25-30 minutes.
Cool and cut into 24 squares.
