In a medium bowl, whisk together the flour and salt. Add the softened butter and use the back of a fork to work it into the flour—just smush and stir until it’s evenly combined and the mixture looks a little crumbly. If needed, use your hands to incorporate it--I often do!
Pour in the sourdough discard and water, then stir with a spatula until you have a shaggy dough. Switch to your hands and gently knead the dough in the bowl until it comes together into a rough ball.
Cover the bowl with plastic wrap or a clean kitchen towel, and let it rest at room temperature for 30 minutes. You can also cold ferment the dough longer for up to 12–24 hours for the best flavor and reduced phytic acid. (The longer you rest the dough, the more complex the flavor. Just be sure to refrigerate it if resting overnight.)
Turn the dough out onto a lightly floured surface and knead for a couple of minutes until it’s smooth and no longer sticking to your hands or the counter.
Cut the dough into 12 pieces for tacos or 6 pieces if you're going for burrito-size. Roll each one into a ball.
Cover them with a towel and let them rest for about 30 minutes before rolling out. This will relax the gluten and allow for a much easier roll out.
After the 30 minute rest, roll each dough ball out on a floured surface until it's nice and thin (about 6 to 8 inches for tacos or 10 to 12 inches for burritos). I like to roll them until I can just start to see through the dough—that’s when I know they’re thin enough.
Heat a dry cast iron skillet (or non-stick pan) over medium-high heat—no oil needed. Lay a tortilla in the hot pan and cook until it starts to puff up a bit and you see those little golden-brown spots underneath. Flip it with your fingers or a pair of tongs or a spatula, and cook the other side just until it gets some color too. Each one should take about a minute total to cook.
Let the cooked tortillas cool, then store in a plastic bag, ziplock bag, or airtight container lined with a tea towel or parchment paper. They stay soft for 2–3 days at room temperature or can be frozen.