Heat oven to 350°F. Grease 13x9-inch baking pan; set aside.
Combine 2/3 cup sugar and 1/2 cup butter in bowl; beat at medium speed, scraping bowl often, until well mixed. Add egg, molasses and 1/2 teaspoon vanilla. Continue beating, scraping bowl often, until creamy. Add flour, cinnamon, ginger, baking soda, cloves and salt; beat at low speed, scraping bowl often, until well mixed.
Press dough into prepared pan. Bake 12-16 minutes or until golden brown. Cool completely. Cut crosswise into 3 equal sections.
Combine 1 cup butter, 1 cup sugar, lemon juice and lemon zest in 2-quart saucepan. Cook over medium heat, stirring occasionally, 2-4 minutes or until mixture just comes to a boil and butter is melted.
Beat 4 eggs with whisk in bowl. Stir small amount of hot lemon mixture into eggs. Whisk egg mixture into remaining hot lemon mixture. Reduce heat to medium-low. Cook, stirring constantly, 4-6 minutes or until mixture reaches 160°F and is thick enough to coat back of metal spoon. Remove from heat; stir in 1 teaspoon vanilla. Refrigerate 2 hours or until completely cooled.
Beat whipping cream in bowl at high speed, scraping bowl often, until stiff peaks form. Gently stir into cooled lemon mixture.
Line 9x5-inch loaf pan with plastic food wrap, extending wrap over edges of pan. Spoon 1 1/3 cups lemon mixture into bottom. Place 1 bar layer over lemon mixture. Repeat layering, ending with lemon mixture. Cover; refrigerate at least 5 hours or overnight.
Remove cover; place serving platter over loaf pan. Flip pan over to release dessert. Remove pan and plastic wrap. Decorate with whipped cream, if desired.