In a large bowl, whisk together the flour, baking powder, salt, and sugar. Gradually add the warm water, mixing until a rough, shaggy dough forms.
Turn the dough out onto a lightly floured surface and knead for 5 minutes, until smooth and elastic. Cover with a damp towel and rest for 20 minutes.
Divide the dough into 6 equal pieces and roll into balls. Roll one ball into a thin round, brush generously with melted butter, then fold into thirds, and fold again into thirds the other direction. Tuck into a ball, cover, and rest 15 minutes. Repeat with remaining dough.
Roll each folded ball into a round about ¼ inch thick and 8 inches across, rolling evenly from the center outward.
Heat a heavy skillet over medium-high heat with a thin layer of oil. Cook each roti about 90 seconds per side, brushing a little extra oil on top before flipping, until golden and blistered.
As each roti comes off the skillet, clap it carefully between your palms a few times to separate the layers. Stack under a clean towel and serve warm.