Add the flour, baking powder, and salt to a large mixing bowl and whisk together until fully combined.
Add your fat of choice and use a fork or pastry cutter to cut it into the flour until the mixture resembles coarse, sandy crumbs. A few larger bits are fine.
Add the room temperature water and sourdough starter and mix until a shaggy dough comes together. Knead for 2 to 3 minutes until the dough is smooth and cohesive. If the dough feels crumbly add water 10 grams at a time. If the dough feels sticky add flour 10 grams at a time. Flour brand and kitchen humidity will affect the dough, so adjust by feel rather than strictly by measurement.
Cover the bowl tightly and let the dough ferment at room temperature for 6 to 8 hours or until noticeably puffed and roughly doubled in size. For the overnight method, mix before bed and let the dough rest covered on the counter for 8 to 10 hours.
Once fermented, punch the dough down and turn it out onto the counter. Divide into 10 to 12 equal pieces, aiming for 60 to 70 grams per ball for a standard tortilla size.
To preshape each ball, make a C shape with your hand around the piece of dough. Push the dough away from you toward the opposite side of your palm, then pull it back down and toward you in a smooth circular motion. Repeat a few times until you have a tight, smooth ball with a taut top. Cover the balls and let them rest for 10 to 15 minutes so the gluten can relax before rolling.
Preheat a heavy bottomed skillet or cast iron pan over medium high heat. Do not grease the pan. A fully preheated dry pan is what produces the characteristic bubble and golden spots.
Working one ball at a time, roll the dough out on a lightly floured surface to approximately 6 to 8 inches across. You can go smaller for taco-sized tortillas or larger for burritos depending on your preference. If the dough springs back, let it rest another few minutes and try again.
Place the rolled tortilla flat onto the hot pan and cook until bubbles form across the surface, approximately 30 to 45 seconds.
Flip and cook for just a few seconds on the second side.
Transfer the finished tortilla to a clean cotton kitchen towel and fold loosely to cover. Repeat with the remaining dough balls. The steam that builds in the towel keeps the tortillas soft and pliable until ready to serve.