Preheat the oven to 180 C. In water bath melt the butter and chocolate. Remove mixture from heat and add sugar, cocoa, salt and vanilla. Mix well. Add the eggs one by one. Add flour and mix until complete mixing of all ingredients. Greased and Flour small rectangular baking dish. Pour the dough and bake 25-30 minutes. Once cool the cake, cut it in 16 squares that you can sprinkle with powdered sugar.