Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13-inch 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 until a toothpick inserted into the center of the cake comes out clean, 45 to 50 minutes. Allow to cool before slicing.

To make the Lemon Sauce: In a saucepan, mix together 2 cups sugar and 4 tablespoons cornstarch. 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, lemon juice, and extract (if desired). Stir in 1 or 2 drops yellow food coloring, if using.

Serve slices of cake with the warm lemon sauce.