Vanilla Custard Cheesecake (OOO) is required, with no substitutions. OOO's VCC is a magic additive/enhancer for bakeries. You won't taste cheesecake or custard at these levels but it thickens up lighter bakery flavors and provides a "backbone" to the recipe without altering the main flavor profile.
Fizzy Sherbet has a powdery texture that helps pull out the grainy textures of the cookie flavors. It makes this recipe sparkle; required.
FA vanilla cookie is the main, required.