It’s Tor-ti-lla. With intonation on the ti. I’m a Spaniard but even if I weren’t, intonation and pronouciation rules are fixed. Tórtilla has intonation on the Tó, and has a tilde because it’s the third syllable. Tortillá has intonation on the llá, which since it ends with an a and has the intonation is on the last syllable, in has a tilde. Tortilla has no tilde, so it must be Tor-TI-lla.