Mini marshmallows are melted into this dark chocolate fudge, giving it the smoothest, creamiest texture.
Lightly brush an 8-inch baking pan with butter, then line with parchment, leaving 2-inch overhang on two sides. Lightly butter parchment.
In a medium saucepan, combine remaining 2 tablespoons butter, sugar, evaporated milk, and salt. Bring to a boil over medium-high heat, stirring constantly, then cook for 3 minutes more, until pale golden and thickened slightly. Remove from heat and stir in marshmallows until mostly melted, then add chocolate and vanilla. Stir until chocolate and marshmallows have melted completely and mixture is smooth. Pour into prepared pan; smooth top with a spatula.
Refrigerate until firm, at least 2 hours. Remove parchment with fudge from pan and cut into 36 squares. Fudge can be stored refrigerated, covered with plastic, up to 1 week.