Slice the hot cross buns into fine slices, then press into the sides of a 1½ L ring-shaped cake mould. Drizzle with rum, then refrigerate for 30 minutes.
Combine the yolks and sugar in a bowl and whisk until very light. Fold in the milk, then cook in a medium saucepan over a low heat until just thickened. Whisk in the gelatine, then strain through a fine sieve.
Melt the dark chocolate and fold in. Cool until just thickened. Whip the double cream and thickened cream to soft peaks and fold in. Spoon into the mould then refrigerate for 2 hours, until firm.
Unmould the cake, arrange on a platter, then surround with speckle eggs.