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Heat oven to 180C/160C fan/gas 4. Butter and line a 20 x 30cm traybake tin with baking parchment. Toss the rhubarb in 50g sugar and set aside to macerate for 20 mins.
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.
Scrape the mixture into the tin. Smooth the surface up to the edges and top with the remaining rhubarb. Bake for 45-50 mins until risen and golden, and a skewer inserted in the centre of the cake comes out clean. If there is any wet mixture, return it to the oven for 5 mins, then check again. Cool in the tin for 10 mins, then serve warm with crème fraîche, or cool completely to serve as a cake. Will keep in an airtight container for three days.