Line a half sheet pan with a nonstick rubber mat. Place a 25cm / 10in by 38cm / 15in by 5-cm / 2-in Plexiglas frame inside the sheet pan. Place the sheet pan in the freezer.
Transfer the parfait base to the prepared frozen frame and spread out evenly using an offset spatula. Place in the freezer to harden.
Line a half sheet pan with a nonstick rubber mat and freeze.
Once the parfait has hardened, flip the parfait onto another nonstick rubber mat. Place it back on the frozen sheet pan and peel off the top rubber mat. Using a 6.25-cm / 2.5-in cutter, cut the parfait out, dipping the square cutter in warm (not hot) water each time to get a clean cut.
Once the parfait is cut, lift each piece using a small offset spatula and transfer to the second prepared frozen sheet pan. If the uncut parfait starts getting too soft, return it to the freezer. The parfait is directly exposed to the freezer ventilation, and this might cause excessive hardening and ice crystal formation, so don’t let it sit in this condition for too long.
Once the squares have hardened, transfer to an airtight container. Reserve frozen until needed. Discard after 3 days.