Bake for 2 hours or until tender when tested with a knife. Smaller potatoes probably will need only 75 minutes but the big ones take longer, so allow ample time just in case.
Make filling:
Brown beef with salt, black pepper, onions, carrots and peas until carrots are tender; drain off excess fat.
Add in marjoram, Worcestershire, beef base and flour, stirring to coat.
Add water and cook until it boils and thickens, then remove from heat.
Potato prep:
Split potatoes in half lengthwise and hollow out centers like canoes using a spoon.
Mash the potato filling and mix with milk, sour cream, salt and pepper and half the melted butter.
Fill potato cavities with meat filling, then spread potatoes over the top.
Top with parmesan, remaining melted butter and paprika.