Cover the sultanas in 250ml of the rum. Stir in the mixed spice and half the vanilla. Leave to soak overnight or 3 hours minimum.
Slice the pears into quarters lengthwise and cut out the core and any pips. Line a tall 23cm cake tin with baking paper and layer the pears in the bottom in a fan shape or whichever way you fancy.
In a saucepan, melt 200g of the butter, the brown sugar and 100ml of rum on a gentle heat until the sugar has melted. Pour two-thirds of the syrup over the pears so they are covered.
Preheat the oven to 150C fan/ gas mark 3½.
Strain the rum-soaked sultanas. In a food processor, blitz the stem ginger and dates, or chop roughly by hand, then mix with the sultanas.
Beat the remaining butter with the caster sugar until pale and fluffy, add the rest of the vanilla and the eggs one by one until combined. Tip in the chopped fruit and beat again. Finally beat in the flour and baking powder until thoroughly combined.
Pile the cake mix into the tin on top of the pears and syrup, and give the tin a gentle shake from side to side to even out the mixture, or use a spatula or back of a spoon, and allow to settle.
Crush the ginger biscuits to a crumb with a rolling pin or in a food processor, and sprinkle over the top of the settled cake mix, then cover with a sheet of tin foil.
Bake in the oven for about 2.5 hours or until a skewer comes out clean when you stick it in the middle.
Remove from the oven and leave to cool in the tin.
Once cooled, run a knife around the edge of the cake, lay a plate over the top, tip it upside down and the whole thing should come out. Gently peel back the paper, pour over the remaining syrup and serve sliced with whipped cream or custard.
