Step 1
Whisk flour salt and baking powder together in a bowl. Make a well in the center. Add egg vinegar oil and warm water. Mix by hand until dough pulls together. Turn dough out onto the counter and knead until soft smooth and elastic. Wrap in plastic wrap and refrigerate for at least 1 hour.
Step 2
Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender about 20 minutes. Drain; transfer to a mixing bowl. Mash and set aside.
Step 3
Chop corned beef and cabbage into fine pieces.
Step 4
Melt butter in a skillet over medium heat. Add onion kosher salt corned beef and cabbage. Cook and stir until onions are soft and golden brown and beef fat is rendered about 10 minutes.
Step 5
Transfer beef mixture into the bowl with the mashed potatoes. Season with salt pepper and cayenne. Mix until combined and let cool to room temperature.
Step 6
Preheat the oven to 375 degrees F (190 degrees C). Line two baking sheets with silicone mats.
Step 7
Divide dough in half; flatten one half into a rectangle on a floured surface. Roll out into a larger rectangle about 1/8-inch thick pulling the corners as needed.
Step 8
Place 3 cups filling over one long end about 3 inches away from the edges. Beat egg with water and brush over the opposite end and a little on the sides. Stretch the dough to cover the filling then roll up toward the egg-washed side. Fold the last 3 or 4 inches on top. Press in the filling; roll over so seam-side is on the bottom.
Step 9
Trim off excess dough from the ends if desired. Mark but do not cut dough into 8 equal pieces. Flour the side of your hand and press into the marks. Rub back and forth against the counter to separate each piece. Position each piece with one cut end facing down and the other facing up. Squish down the top and tuck everything into the center to form a round knish.
Step 9
Repeat with remaining dough and filling. Brush bottoms with olive oil and place on the prepared baking sheets. Brush leftover egg wash on top.