Preheat oven to 350 degrees F (175 degrees C). Grease and flour an 8-inch square pan.
In a large saucepan, melt butter and chocolate over low heat. Remove from heat, and stir in sugar, eggs, and 1 teaspoon vanilla. Beat in 1/2 cup flour and salt. Spread batter into prepared pan.
Bake for 25 to 30 minutes in the preheated oven. Do not overcook.