Combine the flour, baking powder, and salt in a medium bowl. Add the milk and melted butter and knead for 1 minute until a dough forms. The dough should not be sticky and you shouldn't need extra flour for kneading. If it's sticky, add a bit of flour at a time until it's not.
Cover the dough with plastic wrap and let it rest for 20 minutes.
Chop the potato into ½-inch cubes. Add to a medium pot and fill with enough water to cover the potatoes. Bring to a boil. Once boiling, cook for 7-10 minutes until the potato is tender. Drain and run through a potato ricer. Allow the potato to cool to room temperature.
To a bowl, add the potatoes, jalapeno, scallions, mozzarella cheese, cheddar cheese, and salt. Mix until just combined.
Roll the dough into a 10 ½ inch circle.
Use your hands to compress the filling into a 5-inch disc and place in the center of the dough.
Pull the sides up over the dough like you're wrapping a present and pinch the ends together to seal it.
Brush with egg and sprinkle breadcrumbs all over. Flip and do the same for the other side.
Heat 2 tablespoons of oil in a large nonstick pan over medium heat. Add the dough and cook for 4-5 minutes on each side until brown and crispy. Transfer to a cutting board and slice. Enjoy!