Whisk flour and salt together in a medium bowl. Using the back of a fork or pastry cutter, cut the butter into the flour, mixing and smashing it until butter is incorporated. Use your hands to further incorporate the butter into the flour if needed.
Stir in water and sourdough starter and mix with a spatula or hands until a shaggy dough forms. Use your hands to gently knead the dough in the bowl to get the mixture to form a rough ball.
Turn dough out of bowl onto a floured surface and knead for another 1 to 2 minutes or until smooth and not sticking to the surface.
Cut the dough into 10 pieces for taco-sized tortillas or 4 pieces for burrito-sized tortillas. Shape each portion into a ball using your hands and tension of the work surface.
Cover with a very light kitchen towel or plastic wrap on work surface. Let the dough rest for 30 minutes, up to two hours. If you are making the dough in advance, you can store the tortillas balls in an airtight container in the fridge for up to 24 hours.
Using a heavy rolling pin, roll out each ball until very thin (when you start to see the work surface through them.) For taco-sized, the balls should roll out to about 6 to 8 inches. For burrito-sized, about 10 to 12 inches.
Heat a 12 inch pan (cast-iron is best) on medium-high heat. Do not add any oil to the pan. Wait until pan is HOT to start cooking. Lay the tortilla in the pan and cook on first side until it puffs slightly (around 30 seconds). You will hopefully see little brown spots on the underside appear, this means time to flip! Flip with spatula or tongs and cook again till lightly brown. If the skillet is hot enough, cooking time should be about 60 seconds/1 minute total).
Stack tortillas on a plate or in a tortilla holder and cover with a light kitchen towel while you continue making the rest of the tortillas.