Peel the bananas and mash them with a fork in a bowl.
Mix the remaining wet ingredients with the banana puree until you have a smooth paste.
Combine the dry with the wet ingredients.
Line your baking tin (also the sides - alternatively, grease the sides with oil). Pour the batter into the tin.
Trim the ends of the rhubarb and peel the stalks. Cut the stalks into pieces.
Decorate the batter to your liking with the rhubarb pieces. The top should be covered completely with rhubarb pieces. Press the rhubarb pieces gently into the batter (half-way through).
Bake on the middle rack for about 60 minutes, until the top is golden brown.
If you want to enjoy it still warm, be careful when removing the cake from the tin as the sides can be crumbly.
The cake will keep for 5 days in an airtight container.