Line the sides and bottom of a 9-inch cake pan with a single sheet of plastic wrap.
Ladyfingers are rounded on each end. Slice one end of each ladyfinger to make it flat. Save the extra pieces.
Arrange the ladyfingers so they are standing vertically around the sides of the pan, like a picket fence. Lay remaining ladyfingers flat across the bottom, using the cut pieces to fill in any gaps. Your goal is to cover the entire bottom of the pan, which will prevent the filling from oozing out. Set aside.
In a measuring cup, dissolve the cornstarch into milk. A small spoon or a tiny whisk works well.
Fill a medium saucepan with an inch of water and get it simmering.
While it’s heating up, combine egg yolks, 1/4 cup of granulated sugar, and cornstarch-milk mixture in a heat-safe bowl. Place bowl on the top of your saucepan, letting the simmering water below cook your mixture for about 5 minutes until thick and smooth. Be sure your heat is on low, unless you want sweet scrambled eggs!
Remove bowl from heat (use oven mitts). Add lemon juice and zest, and allow to cool.
In a large bowl, cream the butter and powdered sugar with an electric mixer, then add both extracts.
Mix-in lemon/egg mixture until smooth and pale yellow.
In a separate bowl (or stand mixer), whip egg whites into soft peaks, streaming the remaining granulated sugar in on the side of the bowl (so you don't deflate them!); continue whipping until you reach the stiff peak stage.
Fold the egg whites carefully into the butter-lemon mixture. Don't overmix!
Pour half of the filling in the bottom of the pan, then cover with remaining ladyfingers (there should be just enough left for a nice full layer.)
Add the remaining filling and cover with a layer of plastic wrap. Freeze or refrigerate overnight.
When ready to serve, top with whipped cream and garnish with a lemon slice. Remove the cake from the pan by grabbing the bottom layer of plastic wrap and lifting. Set your cake on a platter and gently pull the plastic wrap away from the cake (like a magician pulling off a tablecloth without breaking any plates, but slower). Serve and enjoy!