Line two regular muffin tins with paper cups. Set aside.
If using a rectangular or square pan, line it with aluminum foil, so there is extra foil sticking out on the sides to lift the finished fudge out.
Add the Nutella and butter to a large bowl, microwave and stir the mixture every 40 - 60 seconds, until completely melted and smooth.
Remove bowl from the microwave and stir in the vanilla extract and salt. Mix well with a rubber spatula.
Add the sifted confectioners’ sugar and stir well until completely combined. If using any mix-ins, fold them in and stir well.
Press the fudge into prepared baking pan or paper cups, smoothing the top with the back of a spatula. The top will appear somewhat oily, top with sprinkles. Refrigerate for at least 4 hours or until the fudge is firm to the touch.
Store fudge in an airtight container in the refrigerator for up to 1 week.