Pour about 2 inches of oil into a large pot. Preheat oil to 350-375 degrees.
Pop open can of biscuits and cut each biscuit in half lengthwise. {This is why I use the flaky layers--it's super easy to just peel each biscuit into two equal halves...no knife necessary!} Fry 20-30 seconds per side or until a deep golden brown. Remove from oil and drain on paper towels. Repeat until all doughnuts have been fried.
Place a filling tip into a piping bag and fill with marshmallow fluff. {If you don't have a special filling tip, just use a round, wide mouth metal one.} Fill each doughnut with fluff. Because the store-bought fluff is so thick, use a lot of pressure while filling to make enough come out. You want the doughnuts full!
Once all doughnuts have been filled, microwave hot fudge sauce until warm and loose. Empty out onto plate. Dip the tops of doughnuts into the chocolate and place on cooling rack. Serve.