Marshmallow fluff
Peanut butter filling:
1/2 cup creamy peanut butter
1/8 cup salted butter, softened
3/8 teaspoon vanilla extract
3/4 cup powdered sugar
Cream or milk
Buttercream:
1 cup powdered sugar
1/8 cup salted soft butter
1/4 tsp vanilla
Cream or milk
Chocolate Buttercream:
3/4 cup powdered sugar
1/4 cup cocoa powder
1/8 cup salted soft butter
1/4 tsp vanilla
Cream or milk
Chocolate shell:
Candy melts or melting chocolates
Silicone mold
Squeeze bottles with small tips
piping bags
Prepare any fillings you are making yourself. Whip the ingredients together. Add a small amount of cream or milk as needed to make the filling pipe-able. Adjust ingredients as needed and put them in piping bags.
Shell:
Follow the directions on the package of chocolate or candy melts. (A lot of candy melts suggest using a microwave, but they do better using a double boiler.) You will get the best results if you only make a small amount at a time and make more as needed.
If your mold is pretty flimsy, you will want to put it on a baking sheet.
Use a small brush to paint chocolate all over the inside of the molds; don't worry if there are thin spots at first since you will be repeating this step. Let coated molds harden 15 minutes in the refrigerator.
Repeat the previous steps until you are sure the shell is fairly thick, 2-3 coats total.
Pipe filling into each spot in the mold, but don't fill quite to the top.
Chill in the refrigerator for 10-15 minutes, to avoid melting your filling when you add the chocolate.
Fill a squeeze bottle with chocolate. Keep a bowl or the pan from your double boiler nearby with hot water. When you are not using the bottle, set it in the water to keep the chocolate warm.
Working quickly, fill the remaining space in a few spots, tap the mold on the counter a few times to smooth it out and remove air bubbles, fill a few more, tap it, and so on.
Let the chocolate harden another 15 minutes in the fridge. 
Gently pop the chocolates out of the mold. Use a small knife to clean up edges; if the chocolates start to melt as you work with them, put them in the fridge for a few minutes before continuing.
Store in a cool spot or in the refrigerator.