Line your baking tin with baking parchment.
Pre-heat your oven to 180°C.
In your stand mixer bowl (or equivalent) beat the butter and sugar together until soft.
Add all the remaining ingredients and beat well. Place into your prepared baking tin and smooth to cover the pan evenly.
Now cut the rhubarb slices. These need to be cut diagonally at a 45° angle. With the longer side of your tin towards you, place in rows in a v shape. Don't worry if you don't place the slices correctly the first time, you can pick them out of the mixture and place back again. Continue until you have used all the slices up. If you have any remaining, bake separately.
Bake for 20 minutes in your preheated oven, then check by carefully inserting a knife into the cake mixture on the edge and then in the middle. It is likely to come out clean on the edge, not in the middle. Place back in the oven and cook for a further 7 minutes and check again. It should be ready.
This dessert is best eaten warm from the tin.
