Melt butter and bittersweet baking chocolate (or semisweet chocolate chips) together in a small or medium saucepan over medium low heat, stirring frequently.
¼ cup unsalted butter, 4 ounces bittersweet baking chocolate
Once the chocolate and butter have melted, slowly whisk in the evaporated milk (or half and half if that's what you're using) until combined.
8 ounces evaporated milk
Whisk in the granulated sugar, unsweetened cocoa powder, and salt until combined.
¾ cup granulated sugar, 6 to 8 tablespoons unsweetened cocoa powder, ¼ teaspoon salt
Keep over medium low heat, whisking frequently, about 5 to 6 minutes until mixture comes just to a boil. Reduce heat and simmer for another 2 to 3 minutes, stirring continuously.
Remove from heat and stir in vanilla extract.
2 teaspoons vanilla extract
Serve hot fudge sauce warm over ice cream or allow to cool, transfer to a heat safe jar or container and store in the fridge for up to 3 weeks.