Heat oven to 325°F. Grease bottom and sides of 9-inch square pan with shortening; lightly flour. In large bowl, beat gingerbread ingredients with electric mixer on low speed 30 seconds, scraping bowl constantly. Beat on medium speed 3 minutes, scraping bowl occasionally. Pour into pan.
Bake 50 to 55 minutes or until toothpick inserted in center comes out clean.
Meanwhile, in 1-quart saucepan, mix 1/2 cup granulated sugar and the cornstarch. Gradually stir in 3/4 cup water. Cook over medium heat, stirring constantly, until mixture thickens and boils. Boil and stir 1 minute. Remove from heat. Stir in remaining lemon sauce ingredients. Serve warm or cool.
In chilled medium bowl, beat sweetened whipped cream ingredients on high speed until soft peaks form. Serve lemon sauce and sweetened whipped cream with warm gingerbread.