Prepare the dough by whisking together the flour, salt, and baking powder in a large bowl. In another bowl, combine the vinegar, butter, and 1/2 cup of water. Pour the wet ingredients into the dry and add the egg. Stir with a wooden spoon to get the dough to form, then lightly knead with your hands until you have a well-blended ball. If your dough is too dry, add a little more water. Cover the bowl with a towel and let sit while you make the filling.

For the filling, start by peeling the potatoes and chopping them into large chunks. Put them in a pot and cover with plenty of cold water. Bring to a boil, add in a couple pinches of salt. Cook until almost tender, about 10-12 minutes. Add the broccoli to the pot. Boil for another minute or two, then drain the potatoes and broccoli in a colander. Return the vegetables to the pot.

While the potatoes boil, heat the oil in a small pan. When hot, add the sliced leek. Cook, stirring occasionally, over medium heat. Allow to cook down and start to brown for about 10 minutes. Add to the pot with the drained potatoes and broccoli, along with the sour cream, about half of the cheese, and the salt. Use a large spoon or potato masher to mash everything together. Stir to combine, taste and season with black pepper. Add more salt if it's tasting bland.

Preheat the oven to 375 degrees F and line two baking sheets with parchment paper.

On a clean, floured kitchen counter, roll out half of the dough into an 8 by 16-inch rectangle. You want the long side directly in front of you. Spread half of the filling across the length of the dough, about one inch away from the bottom. Gently roll the dough from the bottom up, covering the filling and wrapping another time or two around. Keep the roll somewhat loose. Trim off the excess dough on either end.

With a ruler, measure every two inches (or your preferred length) down the length of the roll and use a sharp knife to cut the roll at each point. 

Pinch one end of each knish closed and set that end down on the baking sheet. Use the palm of your hand to squish the knish to slightly flatten. Pinch the top to partially close the dough around the top. Do this with each of the remaining pieces. Brush each knish with the beaten egg, top with a little cheese. Repeat this procedure with the second half of dough and filling.

Bake the knishes in the middle of your oven for about 30 minutes or until the pastry is firm and the top is golden brown. Serve warm or at room temperature.