Line a loaf pan with plastic wrap, fill 3/4 full with the ice-cream, cover, and freeze until firm. When hard, use a sharp knife and a soup spoon to dig a trench down the centre of the ice cream, reserving the ice-cream from the trench to make a top later. The trench should leave an inch of vanilla ice cream along with long-ends of the pan and go three-quarters of the way down the pan. Refreeze.
Choose a large metal or glass bowl that will neatly sit on a pot on the stovetop. Put an inch or two (2.5 to 5 cm) water in the pot and bring to a simmer. Meanwhile, beat the yolks and sugar together in the bowl (still on the countertop) with electric beaters until very thick and ribbony. Add the purée and liqueur. Set the bowl over the pot and continue beating until very thick and much increased in volume, about 10 minutes. It should have the consistency of firmly whipped cream. Remove the bowl from the heat and set it over a large bowl full of ice cubes. Continue beating until cool.
Whip the cream to soft peaks and fold it in. Remove the bombe mould from the freezer and pour the raspberry mixture into the well of vanilla ice cream. Cover and return to the freezer for several hours, or until completely set. Soften the reserved “trench” ice-cream and spread it over the top to fill the pan to the brim. Freeze again.
To serve, uncover the mould. Wrap a towel dampened in hot water around the base and hold it upside down. Shake out. Slice with a knife, dipped first in hot water. Refreeze in slice form until ready to serve. Raspberry coulis is the right accompaniment.