Fill a large saucepan with cold water, add potatoes, place on high heat and cover until water comes to a boil.
Once boiling, uncover the saucepan and decrease the heat to a simmer. Cook until potatoes are tender and easily crushed with a fork (around 15 minutes). You can continue on to your meat filling while the potatoes are cooking.
Place the heavy cream and butter in a microwavable bowl, and microwave for around 30 seconds or until warmed through and butter is melted.
Drain the potatoes into a colander, then return them to the saucepan, and mash them.
Add the warmed cream and butter mixture, salt and pepper and mash until smooth. Cool mixture slightly and then stir in the egg yolk until well combined.
Preheat the oven to 200 C (400F).