Preheat you oven to around 180C.
Place the potatoes in a large pan and cover with cold water. Add a little salt, place on the heat and bring to the boil. Turn down and gently simmer for 15-20 minutes.
When the potatoes are cooked (a fork should go through one with little resistance), remove from the heat and drain well.
Place back into the pan and place the pan on a very low heat to try and dry out the potatoes a little before mashing.
Mash the potatoes well until all the lumps are removed. Add the butter and melt in, then add the egg yolk and mix well.
Pipe or spoon the potato on top of the filling.
Sprinkle a little smoked paprika on top and place into the over.
Cook in the oven for 20-25 minutes until the top is nicely browned and everything is cooked through and piping hot.