Butter a square 8 x 8 dish liberally with butter and set aside.
In a heavy bottomed saucepan melt the butter with the sugar and evaporated milk. Bring to a boil then lower the heat and allow to continue to boil on medium-low for 7 minutes stirring constantly, or until soft ball stage. (235-240°F on a candy thermometer)
After 7 minutes add the marshmallows, peanut butter, salt and vanilla. Remove from the heat. Stir vigorously until the marshmallows have melted and completely incorporated. Work quickly.
Pour into the buttered dish and quickly smooth the top with the back of the spoon or an offset spatula.
Sprinkle the chopped peanuts on top pressing gently into the fudge.
Let the fudge cool completely. Place into the fridge to speed up this process.
To prepare chocolate drizzle: Melt 1/2 cup of semi-sweet chocolate chips with 2 Tbsp cream in the microwave in 20 second increments stopping to stir each time. Repeat until smooth and thin enough to drizzle.
Cut fudge into squares, then drizzle with melted chocolate. Refrigerate until chocolate hardens.
Store in an airtight container.
If you place this fudge into the refrigerator to harden, allow it to sit on the counter for 15-20 minutes before attempting to cut into squares.