Preheat oven to 350° F. Coat 8-inch pan with vegetable oil spray or butter lightly.
Melt butter and chocolate over low heat, stirring until smooth. Cool, stir in sugar and vanilla. Add eggs, one at a time, whisking until smooth. Stir in flour and salt.
Pour into prepared pan. Bake in preheated oven 25 to 35 minutes or until top is crusty and cake tester comes out clean when inserted near center.
Remove from oven and cool. When cool, frost, then cut into bars.