To the mashed potatoes, add the yolks and white, and beat them well together. Place it in a pastry bag with a tube having a star-shaped opening (see illustration), and press it through. As the potato comes from the tube, guide it in a circle, winding it around until it comes to a point. The little piles of potato will resemble roses. Touch them lightly with a brush dipped in egg, and place a bit of butter on each one. Put them in the oven a moment to brown slightly. The edges touched by the egg will take a deeper color. Potato roses make a good garnish for meat dishes.