Line a 9x13" pan with parchment paper and set aside.
Place butter, sugar, and evaporated milk in a medium saucepan. Heat over medium heat, stirring constantly, until it starts to bubble.
Once it starts to boil, even just a small boil, set a timer for 3 minutes and 40 seconds. Stir constantly as it bubbles.
Once the timer goes off, remove from heat. Stir in chocolate, until melted and incorporated in. Then add marshmallow cream and vanilla (plus any extracts). Stir vigorously and quickly to incorporate it all. You'll want to stir quickly so it doesn't set up before the marshmallow is incorporated.
Once smooth and well mixed, pour into prepared 9x13" pan. Spread in pan evenly.
If desired, top with crushed toffee, andes mints, candy canes, or any other toppings.
Let set and completely cool for at least 2 hours, then cut into squares and enjoy.
Store in an airtight container at room temperature.