Make the Salted Caramel Sauce

1. Wipe a heavy bottomed medium sized sauce pot clean of all impurities (bits of dust, etc.). Add white sugar in an even layer and cook over medium heat, stirring frequently with a heat resistant spoon while it melts.
2. When the sugar is completely caramelized, quickly whisk in the butter until completely dissolved. Be careful as it will bubble up!
3. Remove from heat and pour in the cream, quickly whisking until smooth. Again, be careful as it will bubble up.
4. Add vanilla extract, and salt. Whisk together until smooth. Set aside in a warm place. Reheat as needed.

Make the Fudge

1. Grease or prepare a 8x8 (for thicker but less fudge squares) or a 9x13 (for thinner but more fudge squares) baking dish with parchment paper. Set aside.
2. In a large sauce pan over medium heat combine sugars, milk, and butter. Bring to a boil and cook for 5 minutes, stirring frequently.
3. After 5 minutes, stir in marshmallows and vanilla extract until completely combined.
4. Stir in milk and dark chocolate until complete combined and smooth. Pour into the prepared baking dish.
5. Swirl 1/2 of the caramel sauce onto the top of the warm fudge with a knife or spoon.
6. Chill fudge until completely cooled and solid. Swirl remaining caramel sauce on top of the fudge and top with sea salt flakes and chopped chocolate as desired. Remove from the pan, cut as desired and serve.