Three layers of an airy gingerbread with a tart lemon filling between them; this is a dependable, all-occasion cake.
To make lemon filling: Have a small bowl ready. Thoroughly whisk together 1/2 cup sugar, eggs, egg whites, lemon zest and lemon juice in a heavy saucepan. Add butter and cook over low heat, whisking constantly, until the mixture has thickened and bubbled several times, about 5 minutes. (The lemon filling must be thoroughly thickened but not allowed to scramble.) Immediately transfer to the bowl. Place a piece of wax paper or plastic wrap directly on the surface to prevent a skin from forming and refrigerate until completely chilled.
To make gingerbread layers: Preheat oven to 350 degrees F. Coat three 9-inch nonstick round cake pans with cooking spray; dust them with flour, tapping out the excess. (If the pans are not nonstick, line the bottoms with circles of parchment or wax paper and lightly oil or spray.)
Sift flour, 3/4 cup sugar, baking powder, baking soda, salt, ginger, cinnamon, allspice and nutmeg into a mixing bowl. Whisk together buttermilk, molasses, egg whites and oil in another bowl; whisk this mixture into the dry ingredients just until blended. Divide the batter among the three prepared pans, spreading it in thin, even layers.
Place the pans on the middle oven rack. (If they will not all fit, place one on the rack below, switching it with another pan midway through baking.) Bake until the tops spring back when lightly touched in the center, 12 to 15 minutes; do not overbake. Let the cake layers cool in the pans for 3 minutes. Turn them out onto wire racks to cool completely, right-side up, and remove the paper, if used.
To assemble cake: Place a cake layer on a serving plate. Spread with half of the lemon filling. Repeat with a second cake layer and the remaining lemon filling. Top with the third cake layer.
Make Ahead Tip: Cover and refrigerate for up to 1 day.
3 other carbohydrate, 1 fat