Mix the butter and sugar together until light and fluffy. Mix the eggs together in a separate bowl, then slowly add to the butter mixture. Fold in the rest of the flour until it has an even consistency.
Divide the mixture in two - add the rhubarb and red colouring paste to one half and cocoa to the other half. Fold the ingredients in.
In two buttered cake tins put large spoonfuls of alternate mixture and tap the tin to even out the mixture rather than mixing or patting the mixture down.
Bake for 30 minutes, until golden brown on top and cooked through Remove from the oven and cool on a rack.
Between the layers use rhubarb jam and finish with icing of your choice.
Recipe courtesy of www.thehappyegg.co.uk