Line 9x13 glass pan with parchment or foil. Let the foil or paper go up the sides of the pan, so you can grab the paper to remove the fudge from the pan. & then spray lightly with cooking spray. Set aside.
In a heavy saucepan, combine sugars, milk and butter. Bring to a low boil, just beginning to bubble and start timer for 5 minutes.
Stir constantly for five minutes, allowing it to come to a full boil. Stir, stir, stir!
Stir in Marshmallow fluff and extract until combined.
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 the paper or foil, lift fudge out of the pan. Cut into squares and store in an airtight container.