Doing some research about the origins of the rum, I found that in the 18th century it was very popular in Nantes, and came in on the trading ships from the Caribbean.
The cake is perfumed, dense, moist and has a hint of almond, and to enhance the rum flavour, the traditional white icing is usually made with rum instead of water.
Preheat oven 180C/gas 4 Grease and base line a 23cm cake tin
Beat 220g of room temperature butter with 250g caster sugar till white and fluffy. Gradually add 80g of sr flour, 200g ground almonds and 4 tbspn rum. Mix together thoroughly. One by one beat in 4 eggs.
Spoon the batter into the tin and bake for 30-40 mins till golden. Cool on a wire rack.
3 tbspn icing sugar mixed with enough rum or water to make a soft but not too runny icing.
Pour over the cooled cake and smooth with a spatula.
As I said, it's a very simple cake, but the rum gives it its unique flavour. If you don't like rum and leave it out, I guess you'd get a routine almond flavoured sponge cake!
My French friend thinks it's best made the day before you want to use it, to let the flavours develop. We ate ours the same day, and it had plenty of flavour!