Pre-heat the oven to 180⁰C, fan 160⁰C, gas 4. Grease 1 x 23cm round spring form cake tin and line with baking paper.
Place a layer of chopped rhubarb at the bottom of the tin and sprinkle half of the caster sugar lightly over the rhubarb.
Whip the 4 egg whites using an electric whisk until soft peaks form, adding the salt and the rest of the caster sugar when bubbles start to appear.
Mix the ground almonds, cinnamon and baking powder in a bowl to form a thick mixture.
In a separate bowl, combine the demerara sugar, 4 egg yolks and the lime zest before adding to the ground almonds mixture.
Carefully fold the egg whites into the cake mixture with a wooden spoon
Pour the cake mixture into the lined cake tin and bake in the middle of the oven for 30 minutes
Leave the cake to cool for 30 minutes before carefully turning it upside down, removing the baking paper and serving with the rhubarb on top.