Start by warming water in a pan, you want about an inch. The actual amount isn't important, especially since saucepan sizes will vary from one house to the next. Bring the water to a nice simmer. Top off with a heat safe bowl. If you have a double boiler, this would be my personal recommendation. Toss in your chocolate, stir until it is smooth and melted completely. Remove from heat to cool a bit.
Prepare your muffin pan with liners. Drop a spoonfull of chocolate into a liner, easiest if you leave the liner in your pan. I tried it in the pan and holding the liner, much easier in the pan but you can try it out just holding the liner in your hand as well if you wish. With the back of your spoon, using the chocolate in the liner, coat the sides and bottom of muffin cup. When you have done this, move on to your next cup. Continue until all of your liners are coated. Chill in the fridge for at least 30 minutes, to set.
Meanwhile, in a medium bowl, mix your peanut butter and sugar until coated well. The combination will be tough to mix, keep at it. When it is blended, add in vanilla and mix well. Set aside.
If you wish to make marshmallow cups, you may make your fluff mixture now.You can actually either use fluff straight or color and flavor it. I decided to add 1/4 tsp strawberry flavor and some pink color to about 1 cup of fluff, since it's Valentine's weekend. You could use up to 1/2 tsp of strawberry flavor without really messing up the consistency of the fluff, this will also depend on how much fluff you mix it with. As with the peanut butter mixture, this will be very hard to mix. Use a fork to combine the ingredients.
Your chocolate cups are waiting. Now what? If you have a small cookie scoop, grab it. This is the perfect size and it will make life much easier. If not, use two spoons and eyeball how much filling you need per cup. You'll want to add enough filling so it's between 1/2 and 3/4 full. Just remember to leave room for the chocolate you still need to top it off with later. If you are making marshmallow cups, microwave the fluff for about 10 seconds to loosen it up. This will make it easier to scoop and fill.
If at this point your chocolate has hardened, heat it up again until it is smooth. Top off each cup, covering your filling. If you have made two or more fillings in one batch, do something to distinguish what is what. I used Valentine colored (red, pink and white) nonpareils for the strawberry marshmallow cups. Once you have finished. Pop your tray in the fridge for a minimum of an hour. When they have hardened completely, remove liner and enjoy.