Sweet potato and smoked gouda perogies

I'm slightly obsessed with perogies, especially after living in Winnipeg which I think is pretty much the perogy capital of Canada. Montreal has alot of good things for sure, the onion soup, the poutine, but good perogies are hard to find around here.
I just tried this new kind of perogy from this cookbook, Rebar, and it's delicious.

Filling:
5 lb (2.3kg) sweet potatoes or yams
2 tbsp butter
3 large leeks chopped, mostly whites
1 tsp salt
1 lb (450g) smoked gouda, grated
cracked pepper to taste

perogy dough
4 cups all purpose flour
3/4 tsp salt
4 egg yolks
1 cup hot water
2 tbsp vegetable oil

Preheat oven to 400 degrees F, scrub the yams then poke them a bit with a fork. Put on an oiled baking sheet wih a bit of water and bake until they're sofy and puffy. Cool them a bit, then mash them.
While the yams roast, heat butter in a frying pan and sautee the leeks until golden and a little caramelized. Add this to your yams along with the cheese, salt and pepper to taste and a little more butter (mm butter). Put this in the fridge.
To make the dough, mix the dry ingredients together. Add the rest and mix with a wooden spoon (or your clean hands) until the dough is smooth, cover it and let it sit at room temp for 30 min.
Divide the dough into quarters, roll it out to about 1/8th of an inch thick (a pasta machine works well for this), then cut out circles with a wine glass, put a tsp or so of filling in the middle, fold it and pinch it closed. Boil the perogies until they rise to the top, toss them with more butter. Serve them of course with sour cream and caramelized onions.
If you want to freeze them, after boiling put one layer on a cookie sheet lined with parchment paper and after theyre frozen stick them into some ziplock bags.
** note: for simpler, traditional, just as delicious perogies you substitute sweet potatoes for white potaotoes, and the gouda for sharp cheddar cheese
Whew!