Sift together flour, salt, sugar, baking powder. Set aside. Bring whey and butter to a rapid boil. Remove from heat and add to flour mixture. Stir with a wooden spoon until a shaggy dough forms, then turn out onto a floured surface and knead until dough comes together and feels smooth. Cover with a towel and let rest for a half hour.
Divide dough in half. Roll each half into roughly 5 equal balls of dough. Let rest, covered, for 20 minutes.
Heat a cast-iron skillet over medium-high heat. Flatten each ball of dough into a 6-inch round. One at a time, place discs into dry skillet until flatbread begins to brown on one side and puff up. Flip and cook for another minute. Hold in a warm bowl lined with a towel. Serve immediately.