1. Preheat oven to 350 degrees Fahrenheit. Grease and flour a 9- by 13-inch baking pan.
2. Melt the chocolate, butter, and espresso mixture in a metal bowl set over a pan of simmering water. Stirring until smooth. Remove the bowl from heat and cool 10 minutes.
3. Stir the cooled chocolate mixture. Whisk in the sugar, vanilla, and eggs one at a time until batter is smooth. Stir in the flour and salt until just combined. Set aside.