Spray a 1-litre terrine mould with oil. Line with plastic wrap, leaving long sides overhanging. Roughly chop 4 bars and place in a heatproof bowl with 300ml cream over a pan of simmering water (don't let bowl touch water) until melted, then stir to combine. Cool slightly, then whisk in remaining cream and egg yolks.

In a separate bowl, beat eggwhites with electric beaters until stiff peaks form. Fold into chocolate mixture. Pour into terrine, folding in wrap. Freeze overnight until firm. To serve, dip mould briefly in warm water, then invert onto a platter. Chop remaining bars, scatter over terrine and drizzle with melted chocolate.