Whisk the flour and salt together in a medium bowl. Cut in the softened butter with a fork until the mixture is crumbly and the butter is well incorporated.
Stir in the water and sourdough starter until a shaggy dough forms. Use your hands to gently knead the mixture into a rough ball.
Knead it for 1–2 minutes until it is smooth and not sticky.
Cut the dough into 16 pieces for taco-sized tortillas or into 8 pieces for burrito-sized tortillas. Roll each piece into a ball. Cover the dough balls with a clean kitchen towel or plastic wrap, and let them rest for 30 minutes to 2 hours.
Roll each ball into a thin round (6–8 inches for tacos, 10–12 inches for burritos).
Heat a dry skillet over medium-high heat. Cook each tortilla for about 30 seconds per side until it puffs up and has brown spots. Adjust the heat as needed.
Stack and cover the cooked tortillas in a clean kitchen towel to keep them soft. Cool the tortillas completely before storing. Keep them in an airtight container at room temperature for up to 5 days, or freeze them for up to 3 months.