How to Make It
Coat bottom and sides of an 8-inch square glass baking dish with butter; line bottom of dish with parchment paper, leaving 4 to 5 inches extending over each end. Coat parchment with butter.
Place chocolate and vanilla in the bowl of a stand mixer. Combine sugar, corn syrup, and 1/2 cup of the half-and-half in a saucepan over medium-high; bring to a boil, stirring until sugar dissolves. Cook, without stirring, until a candy thermometer inserted in mixture registers 230°F, about 15 minutes. Stir in butter, salt, and remaining 3/4 cup half-and-half. Cook, without stirring, until mixture reaches 236°F, about 20 minutes. Pour sugar mixture over chocolate mixture. (Do not stir together.) Let stand until mixture reaches 110°F, about 30 minutes. Beat on medium-low speed until mixture is thick and beginning to become firm, 8 to 10 minutes. Quickly beat in walnuts. Transfer fudge to prepared dish; smooth top. Cover and let stand at room temperature 8 hours or overnight.
Using parchment as handles, remove fudge from dish; cut into squares. Store fudge in an airtight container with parchment or wax paper between layers.