Boil, steam or microwave potatoes until tender, then mash well.
1 kg sebago potatoes
Add 2 tablespoon butter, parmesan cheese, sour cream, parsley (if using) and salt - mixing well to combine.
4 tbsp butter, 40 g parmesan cheese, 1 tbsp sour cream, ¼ cup flat leaf parsley, ¼ tsp cooking salt
Add warm milk to the potato mixture a little at a time, until you have reached your desired consistency (see my notes).
1 cup milk
Place the remaining 2 tablespoon of butter in a small heatproof bowl and microwave until melted (place a sheet of paper/kitchen towel over the bowl to keep your microwave clean).
4 tbsp butter
Add mustards & garlic powder to melted butter and mix well to combine (see my notes).
3 tsp french mustard, 3 tsp whole grain mustard, ¼ tsp garlic powder
Swirl mustard butter through the potato, or dollop on top and serve.