Preheat oven to 350°F. Spray a 9-inch square baking pan with nonstick spray.
For GINGERBREAD CAKE: Mix flour, baking powder, ginger, cinnamon, baking soda and salt in a large bowl. Add molasses, corn syrup, oil and egg; beat until smooth. Beat in water.
Pour batter into prepared pan.
Bake for 35 to 40 minutes or until toothpick inserted in center comes out clean. Cool.
For LEMON SAUCE: Mix sugar and corn starch in small sauce pan. Gradually stir in water.
Cook over medium heat, stirring constantly until mixture boils. Boil and stir for 1 minute; remove from heat. Stir in lemon peel, juice, and butter.
Serve sauce warm or cool with gingerbread cake. Refrigerate any leftover sauce.