http://www.marthastewart.com/341853/chocolate-fudge-filling
Place chocolate chips, sugar, and salt in a medium bowl; set aside. Heat evaporated milk in a small saucepan over medium-high heat until it just comes to a simmer. Remove from heat and pour over chocolate mixture; let stand until chocolate melts, about 5 minutes. Add vanilla, whisk until filling is smooth and well combined.
Transfer chocolate filling to refrigerator; let chill until slightly firm, but still spreadable, about 2 hours.