Combine Dry Ingredients: In a bowl, measure out the wheat flour, corn flour, baking powder, and salt. Mix well.
Incorporate Lard: Add the cold, diced lard to the flour mixture. Use your hands or a mixer with a paddle attachment to blend until the mixture is crumbly.
Add Wet: Make a well in your dry ingredients and add your water and sourdough starter. Mix until incorporated and fold dry ingredients into the wet.
Mix Dough: Pour the warm water sourdough mixture into the crumbly flour mixture. Mix until just combined; don’t worry about over-mixing. The dough will be slightly sticky.
Long Fermentation: Cover the bowl with plastic wrap and place it in the fridge for 12-24 hours for long fermentation. (If you want to make the tortillas right away, simply cover the bowl and let the dough rest on the counter for 15-30 minutes before proceeding.)
Divide the Dough: Once rested, remove the dough from the fridge and divide it into 45-55g balls. Gently roll each piece into a round ball and let them rest for 5-10 minutes.
Preheat the Pan: Once the dough balls are done resting, preheat your cast iron pan or skillet over medium heat.
Roll Out Tortillas: Generously flour your countertop. Roll out a dough ball until very thin and as large as your pan will allow.
Cook Tortillas: Place the flat tortilla onto the hot pan and bake for 30-60 seconds, depending on your pan's heat. Look for golden and brown spots (if it blackens, reduce the heat). Flip the tortilla and bake for an additional 15-30 seconds. Repeat until all tortillas are cooked.
Store or Serve: Wrap the tortillas in tin foil to serve warm, or allow them to cool and store in a plastic bag for later use.