Tip the butter, remaining sugar and zests into a large bowl. Beat with an electric whisk until light and fluffy. Add the eggs, one at a time, mixing well between each addition. Add the flour, polenta, almonds, baking powder and a good pinch of salt, and mix until just combined. Fold through half the rhubarb (reserving the rest for the top), plus any juices, and the marzipan.