Melt the butter in a heavy bottomed saucepan. Add the leeks and turn over in the butter. Add a tablespoon of water and season to taste.
Let the leeks sweat for about 5 mins over a medium heat, stirring from time to time, then add the kale. Stir everything around, add another tablespoon of water and cover again. Leave to sweat for a further 10 mins until both vegetables are soft and buttery. You might need to add a little more water half way through.
Meanwhile cook the potatoes in boiling water until just tender while the greens are sweating. Heat the milk, drain the potatoes then gradually add the milk to the potatoes and mash until smooth.