For the dough: Sift the flour, sugar, salt, and baking powder into a medium mixing bowl. In another bowl, mix together the milk and oil. Make a well in the center of the flour mixture and pour in the liquid mixture. Slowly mix together the dough by working from the center and incorporating the flour from the edges of the 'well', to make a smooth, soft dough. Knead well for 8 to 10 minutes, adding a little flour if the dough is too sticky. Place the dough into an oiled bowl, cover with a damp tea-towel and leave in a warm place for at least 1 hour, or until the dough has doubled in size. Then knock the dough back or punch the dough down, and divide and form into five equal-size balls. Preheat the grill to medium heat. Place a heavy baking sheet on the upper shelf of the grill to heat. On a lightly-floured work surface, using a lightly-floured rolling pin, roll the dough balls out quite thinly, ideally in a teardrop shape, but really this is just aesthetic. Sprinkle over your chosen topping: nigella seeds, poppy seeds, or sesame seeds, or chopped garlic and fresh cilantro leaves, and press into the surface of the dough. Place the naans onto the hot baking sheet and grill for just 1 to 2 minutes, or until lightly browned. Brush the naans with the melted butter and serve hot.