Peel the potatoes and parsnips, cut into similar-size pieces and boil for around 10-15mins until tender to the knifepoint.
Drain the potatoes and parsnips and return to the pan. Roughly chop around half the butter (or a little more) and add to the pan, melting it in the residual heat of the pan and the vegetables.
Mash the vegetables with the butter (if you have a ricer, all the better, but it doesn't need to be a puree). You can leave to cool a bit more at this point if still very hot.
Add the flour, baking powder and a little salt and pepper to taste. Mix well - it should come away from the pan into a ball. If it's too wet, add a little more flour. If it won't come together, add a little milk.
Remove the mixture to a floured surface, press flat into a circle around ⅓in/1cm thick and cut into four (or divide and make eight, if you prefer thin).
Heat the remaining butter in a skillet/frying pan over a medium heat and cook around 3minutes on each side until golden then serve.