Potato Cakes For Brunch or Lunch or Dinner

I've been experimenting/foolproofing/testing recipes for Christmas Eve, Christmas morning and and I found this recipe is extremely tasty and versatile enough to be served as a breakfast or dinner side dish. My 10 year old niece was my sous chef last night and she decided that the results deserved a "quadruple gold star."

Potato & Onion Cakes

2 tablespoons unsalted butter, softened, plus more
to grease the tin
Coarse salt and freshly ground pepper
1 tablespoon packed dark-brown sugar
1 tablespoon balsamic vinegar
1 tablespoon red wine vinegar
2 teaspoons finely chopped fresh rosemary, plus 6
sprigs for tin
1 small red onion, sliced into six 1/4-inch-thick
rounds
4 medium Yukon gold potatoes, coarsely grated
1 large egg yolk, lightly beaten

1. Preheat oven to 400°. Generously butter a
large (six 1-cup) muffin tin. Sprinkle each cup with
salt, pepper, and 1/2 teaspoon sugar; drizzle each
cup with 1/2 teaspoon of each vinegar. Put a
rosemary sprig in each cup, and cover with an onion
round; set aside.

2. Toss potatoes with chopped rosemary and egg
yolk in a medium bowl; season with pepper and 2
teaspoons salt. Divide among cups; dot with butter.

3. Bake until potatoes are tender and well
browned, about 30 minutes. Remove from oven. Let
Cool 5 minutes. Run a thin knife around edge of each
cup to loosen; invert, and serve.