Line a 9" x 13" baking pan with foil and then grease the foil with butter or cooking spray.
In a large saucepan, melt together butter and chocolate over low heat, stirring frequently and being careful not to burn. I put mine on the lowest temperature and actually turn the burner off when the chocolate is about 3/4 of the way melted and just keep stirring until it is completely melted.
Remove from heat and stir in sugar and vanilla.
Slowly add eggs, stirring constantly and continue to stir until all of the eggs have been incorporated. This will take several minutes - just keep stirring.
Add the flour and stir until all the flour is incorporated.
Pour into prepared pan.
Bake for approximately 30 minutes or until center is almost set.
Let cool completely before cutting.
Recipe by From Calculu∫ to Cupcake∫ at https://www.fromcalculustocupcakes.com/best-basic-brownies/