lilmod, the story about Rav Shach and the two seminary girls is brought down in the Sefer Lulei Sorasecha. You are correct it was a Ramban (not Rambam) they asked him about, but it was kugel not baking that he answered them with.
After the girls asked him about the Ramban, Rav Shach in his trademark warmth and smile asked them if they knew how to make kugel. They affirmed they did. And Rav Shach replied “And I know how to learn yet must still spend my life working on learning better. Your job is to work on making better kugel.”