Prepare the dough. Stir together flour, salt, and baking powder in a large mixing bowl. Using a pair of chopsticks or spatula, stir the flour while adding the milk until incorporated. You may have dry spots of flour left in the bowl.
Add the softened and use your hand to knead it into a soft dough. (Since this is a soft dough, I used a glove to prevent sticking). Cover the dough with a damp paper towel and cover the bowl with a lid. Refrigerate for about an hour or overnight.
Prepare the filling. Clean a potato and punch a few holes with a fork. Place in a pot and cover with enough water. Boil until tender, about 30-45 minutes. Transfer the potato to an ice bath to cool down. Then, peel off the skin. Mash the potato in a bowl until no lumps appear. Season with nutritional yeast, salt (I used ¼ teaspoon), chopped scallions, and vegan mayo. Mix to combine. Taste test and season accordingly. Please note that some vegan cheese is saltier, so adjust the taste as needed.
Lightly dust a work surface with flour. Remove the dough from the refrigerator.
Roll it out into a large thin circle, about 12 to 14 inches in diameter. Shred about ½ cup of vegan cheese in the middle of the dough. Then, top with the seasoned mashed potatoes and more cheese if needed.
Lift up the sides of the dough and start to pleat to seal towards the middle to close all the openings (see video).
Then, gently flatten it using a rolling pin into a 10 to 12 inches diameter bread.
To cook the bread, melt a tablespoon of butter in a hot skillet. Place the bread in, seamed side down. Brush the top with melted vegan butter. Pan-fry until each side is deeply golden, flipping a few times if necessary.
Serve warm as-is or with a side of chili oil.