In a large bowl, add flour, baking powder, and salt, and mix them together. Add in the melted butter and warm water, then mix together until a smooth ball forms. Stop mixing as soon as a ball forms to prevent overmixing.
Allow the dough to rest in the bowl, covered with plastic wrap, for 20-30 minutes at room temperature.
Flour a smooth surface and use a rolling pin to roll the dough to ¼-inch thick. Then use a sharp knife to cut the dough into rectangles.
Heat the oil in a large cast-iron pot over medium heat to 375°F. Test one sopapilla in the oil first. It should cook slowly, puffing up from the inside and browning on the outside. Cook 4-6 sopapillas at a time, making sure you don’t overcrowd the pot. Once golden-brown, take them out carefully with a slotted spoon and drain them on a paper towel.