In a large bowl, combine the sugar and lukewarm water. Stir until the sugar dissolves completely. Sprinkle the yeast over the water and let it bloom for 2 to 3 minutes. If the yeast doesn't become foamy, discard it and start again with fresh yeast.
Add the atta, yogurt, oil, and salt to the bloomed yeast mixture. Using your hands, gently work the flour into the wet ingredients until everything is evenly combined. At this stage, the mixture will resemble wet sand.
Gradually pour in the warm milk while mixing with your hands or a spatula. Start with ⅓ cup milk. Continue until the flour is fully hydrated, adding only as much milk as needed. Depending on quality of your atta, you may not need to use all that the quantity I mentioned above.
Bring the dough together into a soft, slightly sticky ball. It will feel softer than regular roti dough, and that's exactly what you want. As the dough rises, the atta will absorb the extra moisture and become much easier to handle. Starting with a softer dough is the secret to making naan that stays soft instead of turning dry.
Lightly oil your hands and smooth the surface of the dough. Transfer it to a lightly greased bowl(large enough to contain the risen dough), cover with a kitchen cloth or tight fitting lid, and let it rise for 2 to 4 hours, or until doubled in volume.
Once the dough has doubled, gently punch it down to release the trapped air. Transfer it to a lightly floured work surface and divide it into equal portions.
Shape each portion into a smooth ball by tucking the edges underneath. Cover the dough balls with a clean kitchen towel and let them rest for 15 to 20 minutes. This short rest relaxes the gluten, making the dough much easier to roll without springing back.
Lightly dust your work surface with atta and place one dough ball on it. Roll it into an oval or teardrop shape about ¼ inch thick. Don't worry about making it perfectly even. A slightly rustic shape is part of homemade naan's charm. If the dough resists or shrinks back while rolling, let it rest for a few minutes before continuing.
Heat a cast iron tawa or skillet over medium high heat until very hot. Place the rolled naan onto the hot surface and cook for 1 to 2 minutes, or until bubbles appear all over the top and the underside develops golden brown spots.
Flip the naan and cook the other side until lightly charred in places and cooked through. Adjust the heat as needed so the naan cooks evenly without burning before the center is done.
Optional Step - For crispier naan, transfer it directly to an open gas flame for 10 to 20 seconds, flipping once or twice, until lightly charred around the edges and crisp to your liking. Watch it closely to prevent burning.
Brush the hot naan generously with melted butter or ghee and serve immediately.