This recipe moves fast so have all of your ingredients measured and ready to go before you get started!
Line an 8x8-inch baking pan with parchment paper and spray well with nonstick spray. Set aside (close by).
In a medium saucepan (2-quart size) set over medium heat, heat the evaporated milk, sugar, and salt together until boiling (stirring constantly). Let boil over medium heat, still stirring constantly, for 5 minutes. Don’t remove from heat before the 5 minutes are up or the fudge will not firm up later during chilling.
⅔ cup evaporated milk, 1 cup granulated sugar, ¼ teaspoon kosher salt
Remove the saucepan from the heat and immediately stir in the semisweet chocolate chips, dark chocolate chips, and butter. Stir until fully melted.
¾ cup semisweet chocolate chips, ¾ cup dark chocolate chips, ¼ cup unsalted butter
Stir in marshmallow fluff and vanilla extract until fully combined.
4 ounces marshmallow fluff, 1 teaspoon pure vanilla extract
Use a spatula to place all of the fudge mixture into the prepared baking dish.
Let chill in fridge for at least 5 hours or overnight before slicing into squares.