Dissolve the gelatin in the cold water and set aside.
Combine milk, cream, sugar and liqueur in a saucepan. Bring to a quick boil and
remove from heat.
Temper the gelatin with a small amount of hot mixture then add gelatin to the
remaining hot liquid. Pour into a 10-cup mold or fluted pan and refrigerate until
set; at least 8 hours, preferably overnight.
To unmold the panna cotta, briefly dip the mold in hot water and carefully flip
it over on a plate.