Storage: Allow the squares to cool completely in the pan before cutting. Store leftovers covered at room temperature for up to 2 days, or refrigerate for up to 5 days. Reheat gently before serving.
Substitutions: You can substitute real butter for the margarine for a richer flavor. Any firm baking apple variety, such as Granny Smith or Honeycrisp, works well.
Tips: If the bottom crust feels too fragile to handle, keep it in the refrigerator until just before adding the filling. For a crispier top, increase the baking time by 5-10 minutes if needed, checking frequently.