Preheat oven to 180 degrees C. Grease a 23cm springform tin and sprinkle it with a little flour.
Polenta Cake: Beat the butter and sugar with an electric beater until foamy. Whisk the eggs in a separate bowl then fold into the butter and sugar.
Sift flour and mix with polenta and baking powder. Mix into the dough. Mix in the rum, lemon juice, orange juice, lemon zest and orange zest.
Pour the mixture into the tin and smooth out. Bake in the preheated oven for 50-60 minutes until a toothpick comes out clean. If the cake is not cooked through, but the top is too dark, cover with a piece of aluminium foil.
Remove from the oven, let cool slightly and remove from the tin. Cool completely on a wire cake rack.
For the sauce, mix 500ml orange juice with the marmalade and reduce in a small pan without a lid.
Before serving, pour the sauce over the pieces of cake (the sauce tastes good hot or cold).