Rub margarine around the inside of an 8-inch round springform cake tin and line with baking parchment. Heat the oven to 180C/160C fan/ gas 4. Tip the raisins and rum into a blender and blitz to create a thick paste (there will be a few lumps).
Beat the 225g margarine and sugar together in a separate bowl using a whisk or electric whisk until pale and fluffy, then add the eggs, one at a time, beating after each addition. Don’t worry if the mixture splits – it will eventually come together.
Sift in the flour and cocoa, and mix using a spatula until just combined. Mix in the rum and raisin paste.
Spoon the batter into the prepared tin and bake for 35 mins until a skewer inserted into the middle comes out clean. If wet batter still clings to the skewer, bake for a further 10 mins, covering with foil if the cake has browned too much. Leave to cool in the tin for 15 mins before removing to a wire rack to cool completely. The cake can be served straightaway or fed with 1-2 tbsp rum weekly, wrapped well and stored in an airtight container for up to a month.
