Preheat the oven to 180°C and grease and line a 20cm x 20cm cake tin.
Dissolve the sugar with 100ml of water in a saucepan and bring to the boil, stirring occasionally until it has completely dissolved. Reduce to a simmer then add the rhubarb and poach for 2-3 minutes. Remove and strain the rhubarb then reserve to one side.
In a large mixing bowl, beat together all the ingredients for the cake apart from the bananas using an electric hand-held whisk until smooth; usually 1-2 minutes.
Mash the bananas in a separate bowl using a fork until smooth. Fold into the cake batter until incorporated. Spoon the batter into the prepared cake tin and smooth until even. Sprinkle the ground almonds on top in an even layer. Arrange the rhubarb pieces on top of the ground almonds in a crisscross pattern and bake for 20-25 minutes until a cake tester comes out clean from the centre of the cake.
Remove and allow to cool in the tin before turning out on a chopping board and cutting half the cake into 5cm x 2.5cm slice before serving.