Try to choose the same size potatoes so they are cooked simultaneously. If the size of the potatoes is too different, then cut the largest ones into several parts. Robuchon made his purée with a manual food mill. If you are lucky enough to have such a device, I guarantee that your purée will be smooth. Never use warm butter. It is important that the oil is cold: then the texture of the purée will be so smooth and velvety.