Preheat oven to 180 degrees C. Grease and flour a 20x30cm pan. Sift together the flour, baking powder, salt, cinnamon, nutmeg and cloves. Set aside.
In a large bowl, cream together 2 tablespoons butter and 1 cup sugar until light and fluffy. Beat in the egg. Beat in the flour mixture alternately with the milk, mixing just until incorporated. Stir in the chopped rhubarb. Pour batter into prepared pan.
Bake in the preheated oven for 45 to 50 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool. Serve with Lemon Sauce.
To make the Lemon Sauce: In a saucepan, mix together 2 cups sugar and 4 tablespoons cornflour. Stir in the water. Cook over medium heat, stirring constantly, until mixture is thickened. Remove from heat and stir in 4 tablespoons butter, lemon zest and lemon juice. Stir in 1 or 2 drops yellow food colouring if desired.