Preheat oven to 180 degrees C. Grease and flour a 20x30cm pan.
In a large bowl, mix dates, 1 teaspoon soda and boiling water. Cool.
Add flour, 1 cup sugar, mayonnaise and remaining 1 teaspoon soda. Using medium speed of mixer beat until mixture is well blended. Stir in nuts. Spread batter in prepared pan.
Bake for 45 minutes, or until a toothpick inserted in centre comes out clean.
While the cake is baking, prepare the lemon sauce. Mix beaten egg and 1/4 cup (65ml) water; place in a medium saucepan. Add 1 cup sugar, butter or margarine and lemon peel and juice. Heat to boiling over medium heat, stirring constantly. Serve sauce over warm cake. Refrigerate any leftover sauce.