With a wooden spoon, add in some of the cream and beat vigorously to combine it all. Remember, the final amount of cream that your will add to this will depend on how starchy the potatoes actually are and this can vary from potato to potato, if albeit slightly. Anyway, I would start by adding ½ cup of cream before plopping in a few tablespoons of butter, cut into pads, and continuing to beat them in. Continue to add cream and butter, beating all the while, until you’re happing with the consistency. Remember that these don’t have to be smooth, and in fact, shouldn’t be; they should have some soft, nubbly lumps of potatoes in there.