Line a 9 x 13 pan with foil or parchment paper. You want the edges of the foil or paper to go up the sides of the pan, so you can grab them to remove the fudge from the pan. Spray paper or foil with cooking spray
In a heavy saucepan, combine sugars, milk and butter. Bring to low boil, just beginning to bubble and start timer for 5 minutes. Stir constantly for the 5 minutes, allowing it to come to a full boil. Stir, stir, stir!
Stir in marshmallow fluff and vanilla until blended.
Add in chocolate and stir until melted. Immediately pour into prepared pan.
Shake or tap the pan to get the surface of the fudge smoother, avoid touching it with your hands.
Allow to set up at room temperature.
Using foil or paper, lift fudge out of pan. Remove foil or paper and cut fudge into squares. Store in an airtight container with layers of paper between fudge.