Put 260 grams of the flour (about 2 cups) in the bowl of a stand mixer bowl with the salt, sugar, baking powder and instant yeast. Stir well, then add sour cream, warm milk, warm sweet potatoes and oil and mix the best you can by hand or use the paddle.
Attach dough hook and begin kneading the dough with the hook. The dough will seem dry at first, but it should come together and form a slightly smooth ball as you knead. The potatoes will keep it from being perfectly smooth, but it shouldn't stick to your fingers. If the dough is too sticky, add a little more flour. If it is too dry, you can add a little more milk.
Once the dough is smooth and elastic you can simply cover the bowl with plastic or if you prefer, transfer the bowl to a second lightly greased bowl.
Allow dough to rise for about 45 minutes or until doubled in bulk.
Punch down the dough and divide it into 6 sections. Shape each section into a ball, then flatten the balls into large rounds or slightly triangular shapes. Use your fingers to make little dimples in the dough.
Heat a large cast iron skillet. Add a one of the dough pieces and cook for about 40 seconds to 1 minute. Flip and cook the other side for another 40 seconds. Continue cooking and turning (and controlling the heat) until the dough is well done on the outside and cooked through on the inside. If you have a thermometer, you can poke it in the center to make sure the naan is about 205 degrees F, but after making a couple you'll get the feel for how long it takes.
Remove from skillet and set cooked naan on a plate or on a toaster oven tray lined with foil if you wish to keep it warm. Brush naan with melted butter or ghee.