Prepare a 9x13 pan by lining it with parchment paper or spraying with non-stick cooking spray. I love the new double sided foil parchment because it's so much easier to shape than just regular parchment.
Spray the inside of a 3-qt heavy saucepan with cooking spray. On medium heat, begin warming the butter, corn syrup and evaporated milk. Add in the sugar. Stir gently until mixture is combined and sugar begins to dissolve. Continue cooking over medium heat for another 20 minutes- until all the sugar is dissolved.
While the fudge is cooking, make sure you have your mint chips and semi sweet chips measured out. To create the two layers, you'll eventually separate the warming candy into 2 parts. I like to put the mint chips in a glass bowl (something that retains heat well.) The semi-sweet chips will go into the pan (after you pour out half of the mixture.)
After about 20 minutes the mixture will begin to boil. Boil for about 6-8 minutes, until candy thermometer reaches 234 degrees F. (If you don't have a candy thermometer, it's okay- just set the timer to 7 minutes once the mixture begins boiling.)
Remove pan from heat and immediately stir in the marshmallow creme. Stir gently until it's completely incorporated. Now, you're going to divide the mixture into 2 parts- half is going to go into the bowl with the mint chips. The other half will remain in the pan, where you'll add the semi-sweet chips and the vanilla. Pour half in with the mint chips and stir several times- it doesn't have to be completely incorporated- then let it sit while you focus on the semi-sweet part.
Add the semi-sweet chocolate chips and 1 tsp of vanilla to the remaining candy. Stir gently until the chocolate is completely melted and incorporated into the rest of the candy. The fudge will begin to set, so once it's all incorporated, pour into your prepared pan and press gently to smooth out and extend fudge across the whole pan.
Now let's focus on the mint fudge. You'll probably find that it's beginning to set up a bit. Stir a few times, then stick it in the microwave for 30 seconds, just to soften it up again so you can melt the chips and combine with the rest of the mixture. Once it's all combined, pour it over the top of the semi-sweet fudge layer, using a rubber scraper to gently press the mint fudge into the pan and smooth it out over the entire layer of semi-sweet.
Sprinkle fudge with holiday candies and gently press in with a rubber scraper. Allow fudge to cool for at least an hour before cutting into small squares.
