Best frozen brake discs I ever saw were on a Passatt. Car was left in -24C overnight, then brought into a workshop at around 18C, but also quite humid relatively because of the melting snow and ice off other cars. It stayed in the workshop for about 3 minutes. This wasn't long enough for the discs to warm up, but was long enough for a nice film of condensation to form on the discs, then freeze. Result, zero braking and a lovely dint in a roller shutter door.