Melt the butter and chocolate in a medium saucepan over low heat, stirring until smooth. Cool for 2 minutes. Stir in the sugar and vanilla. Add the eggs, one at a time, whisking until smooth. Stir in the flour and salt. Pour the mixture into the pre-pared pan; spreading evenly. Bake for 25 to 35 minutes or until the top is crusty and a cake tester inserted near the center comes out clean. Cool to room temperature.