Prepare a slice tin (25 x 25cm approx) with baking paper along the bottom and sides
In a large sauce pan add the marshmallows and cubed butter and begin to heat over a low temp.
Stir constantly with a rubber spatula or wooden spoon until the marshmallow is fully melted and the butter incorporated, be sure to not let it catch on the side and bottom of the pot.
Remove from the heat and add the rice bubbles and mix quickly and thoroughly (if adding choc bits etc add them now).
Pour into the prepared slice tin and press down with a large wooden spoon or spatula as best you can.
To get a smooth finish use a large piece of baking paper over the top and smooth the slice with your hands until flat.
If using can add the sprinkles over the top of the slice now.
Allow to cool to room temperature before slicing, once completely cold store in an airtight container.