Place ice-cream in a bowl and stand at room temperature until softened slightly.
Scoop pomegranate arils into a sieve over another bowl to catch the juice.
Add 3/4 of the arils and vanilla extra to the ice-cream and fold through.
Transfer ice-cream to a 2L capacity, plastic wrap-lined mould. Freeze overnight.
Pour pomegranate juice, leftover arils and sugar into a non-reactive saucepan. Bring to the boil and add dissolved arrowroot. Simmer until thick. Stand to cool to room temperature. (Can be prepared a day ahead).
When ready to serve, melt chocolate over a double boiler.
Turn terrine upside down onto serving plate, remove wrap and slice.
Serve ice-cream drizzled with chocolate and pomegranate syrup.